PHP Filter

PHP filters are used to validate and filter data coming from insecure sources, like user input.

What is a PHP Filter?

A PHP filter is used to validate and filter data coming from insecure sources.

To test, validate and filter user input or custom data is an important part of any web application.

The PHP filter extension is designed to make data filtering easier and quicker.

Why use a Filter?

Almost all web applications depend on external input. Usually this comes from a user or another application (like a web service). By using filters you can be sure your application gets the correct input type.

You should always filter all external data!

Input filtering is one of the most important application security issues.

What is external data?

• Input data from a form
• Cookies
• Web services data
• Server variables
• Database query results

Functions and Filters

To filter a variable, use one of the following filter functions:

• filter_var() - Filters a single variable with a specified filter
• filter_var_array() - Filter several variables with the same or different filters
• filter_input - Get one input variable and filter it
• filter_input_array - Get several input variables and filter them with the same or different filters

In the example below, we validate an integer using the filter_var() function:

<?php $int = 123; if(!filter_var($int, FILTER_VALIDATE_INT))   {   echo("Integer is not valid");   } else   {   echo("Integer is valid");   } ?>

The code above uses the "FILTER_VALIDATE_INT"  filter to filter the variable. Since the integer is valid, the output of the code above will be: "Integer is valid".

If we try with a variable that is not an integer (like "123abc"), the output will be: "Integer is not valid". For a complete list of functions and filters, visit our PHP Filter Reference.

Validating and Sanitizing

There are two kinds of filters:

Validating filters:

• Are used to validate user input
• Strict format rules (like URL or E-Mail validating)
• Returns the expected type on success or FALSE on failure

Sanitizing filters:

• Are used to allow or disallow specified characters in a string
• No data format rules
• Always return the string

Options and Flags

Options and flags are used to add additional filtering options to the specified filters.

Different filters have different options and flags.

In the example below, we validate an integer using the filter_var() and the "min_range" and "max_range" options:

<?php $var=300; $int_options = array( "options"=>array   (   "min_range"=>0,   "max_range"=>256   ) ); if(!filter_var($var, FILTER_VALIDATE_INT, $int_options))   {   echo("Integer is not valid");   } else   {   echo("Integer is valid");   } ?>

Like the code above, options must be put in an associative array with the name "options". If a flag is used it does not need to be in an array.

Since the integer is "300" it is not in the specified range, and the output of the code above will be: "Integer is not valid".

For a complete list of functions and filters, visit our PHP Filter Reference. Check each filter to see what options and flags are available.

Validate Input

Let's try validating input from a form.

The first thing we need to do is to confirm that the input data we are looking for exists.

Then we filter the input data using the filter_input() function.

In the example below, the input variable "email" is sent to the PHP page:

<?php if(!filter_has_var(INPUT_GET, "email"))   {   echo("Input type does not exist");   } else   {   if (!filter_input(INPUT_GET, "email", FILTER_VALIDATE_EMAIL))     {     echo "E-Mail is not valid";     }   else     {     echo "E-Mail is valid";     }   } ?>

Example Explained

The example above has an input (email) sent to it using the "GET" method:

1.     Check if an "email" input variable of the "GET" type exist

2. If the input variable exists, check if it is a valid e-mail address

Sanitize Input

Let's try cleaning up an URL sent from a form.

First we confirm that the input data we are looking for exists.

Then we sanitize the input data using the filter_input() function.

In the example below, the input variable "url" is sent to the PHP page:

<?php if(!filter_has_var(INPUT_POST, "url"))   {   echo("Input type does not exist");   } else   {   $url = filter_input(INPUT_POST,   "url", FILTER_SANITIZE_URL);   } ?>

Example Explained

The example above has an input (url) sent to it using the "POST" method:

1.     Check if the "url" input of the "POST" type exists

2.     If the input variable exists, sanitize (take away invalid characters) and store it in the $url variable

If the input variable is a string like this "http://www.W3ååSchøøools.com/", the $url variable after the sanitizing will look like this:

http://www.W3Schools.com/

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Filter Multiple Inputs

A form almost always consist of more than one input field. To avoid calling the filter_var or filter_input functions over and over, we can use the filter_var_array or the filter_input_array functions.

In this example we use the filter_input_array() function to filter three GET variables. The received GET variables is a name, an age and an e-mail address:

<?php $filters = array   (   "name" => array     (     "filter"=>FILTER_SANITIZE_STRING     ),   "age" => array     (     "filter"=>FILTER_VALIDATE_INT,     "options"=>array       (       "min_range"=>1,       "max_range"=>120       )     ),   "email"=> FILTER_VALIDATE_EMAIL,   ); $result = filter_input_array(INPUT_GET, $filters); if (!$result["age"])   {   echo("Age must be a number between 1 and 120.<br />");   } elseif(!$result["email"])   {   echo("E-Mail is not valid.<br />");   } else   {   echo("User input is valid");   } ?>

Example Explained

The example above has three inputs (name, age and email) sent to it using the "GET" method:

1.     Set an array containing the name of input variables and the filters used on the specified input variables

2.     Call the filter_input_array() function with the GET input variables and the array we just set

3.     Check the "age" and "email" variables in the $result variable for invalid inputs. (If any of the input variables are invalid, that input variable will be FALSE after the filter_input_array() function)

The second parameter of the filter_input_array() function can be an array or a single filter ID.

If the parameter is a single filter ID all values in the input array are filtered by the specified filter.

If the parameter is an array it must follow these rules:

• Must be an associative array containing an input variable as an array key (like the "age" input variable)
• The array value must be a filter ID or an array specifying the filter, flags and options

Using Filter Callback

It is possible to call a user defined function and use it as a filter using the FILTER_CALLBACK filter. This way, we have full control of the data filtering.

You can create your own user defined function or use an existing PHP function

The function you wish to use to filter is specified the same way as an option is specified. In an associative array with the name "options"

In the example below, we use a user created function to convert all  "_" to whitespaces:

<?php function convertSpace($string) { return str_replace("_", " ", $string); } $string = "Peter_is_a_great_guy!"; echo filter_var($string, FILTER_CALLBACK, array("options"=>"convertSpace")); ?>

The result from the code above should look like this:

Peter is a great guy!

Example Explained

The example above converts all "_" to whitespaces:

1.     Create a function to replace "_" to whitespaces

2.     Call the filter_var() function with the FILTER_CALLBACK filter and an array containing our function

PHP Exception Handling

Exceptions are used to change the normal flow of a script if a specified error occurs

What is an Exception

With PHP 5 came a new object oriented way of dealing with errors.

Exception handling is used to change the normal flow of the code execution if a specified error (exceptional) condition occurs. This condition is called an exception.

This is what normally happens when an exception is triggered:

• The current code state is saved
• The code execution will switch to a predefined (custom) exception handler function
• Depending on the situation, the handler may then resume the execution from the saved code state, terminate the script execution or continue the script from a different location in the code

We will show different error handling methods:

• Basic use of Exceptions
• Creating a custom exception handler
• Multiple exceptions
• Re-throwing an exception
• Setting a top level exception handler

Note: Exceptions should only be used with error conditions, and should not be used to jump to another place in the code at a specified point.

Basic Use of Exceptions

When an exception is thrown, the code following it will not be executed, and PHP will try to find the matching "catch" block.

If an exception is not caught, a fatal error will be issued with an "Uncaught Exception" message.

Lets try to throw an exception without catching it:

<?php //create function with an exception function checkNum($number)   {   if($number>1)     {     throw new Exception("Value must be 1 or below");     }   return true;   } //trigger exception checkNum(2); ?>

The code above will get an error like this:

Fatal error: Uncaught exception 'Exception' with message 'Value must be 1 or below' in C:\webfolder\test.php:6 Stack trace: #0 C:\webfolder\test.php(12): checkNum(28) #1 {main} thrown in C:\webfolder\test.php on line 6

Try, throw and catch

To avoid the error from the example above, we need to create the proper code to handle an exception.

Proper exception code should include:

1.     Try - A function using an exception should be in a "try" block. If the exception does not trigger, the code will continue as normal. However if the exception triggers, an exception is "thrown"

2.     Throw - This is how you trigger an exception. Each "throw" must have at least one "catch"

3.     Catch - A "catch" block retrieves an exception and creates an object containing the exception information

Lets try to trigger an exception with valid code:

<?php //create function with an exception function checkNum($number)   {   if($number>1)     {     throw new Exception("Value must be 1 or below");     }   return true;   } //trigger exception in a "try" block try   {   checkNum(2);   //If the exception is thrown, this text will not be shown   echo 'If you see this, the number is 1 or below';   } //catch exception catch(Exception $e)   {   echo 'Message: ' .$e->getMessage();   } ?>

The code above will get an error like this:

Message: Value must be 1 or below

Example explained:

The code above throws an exception and catches it:

1.     The checkNum() function is created. It checks if a number is greater than 1. If it is, an exception is thrown

2.     The checkNum() function is called in a "try" block

3.     The exception within the checkNum() function is thrown

4.     The "catch" block retrives the exception and creates an object ($e) containing the exception information

5.     The error message from the exception is echoed by calling $e->getMessage() from the exception object

However, one way to get around the "every throw must have a catch" rule is to set a top level exception handler to handle errors that slip through.

Creating a Custom Exception Class

Creating a custom exception handler is quite simple. We simply create a special class with functions that can be called when an exception occurs in PHP. The class must be an extension of the exception class.

The custom exception class inherits the properties from PHP's exception class and you can add custom functions to it.

Lets create an exception class:

<?php class customException extends Exception   {   public function errorMessage()     {     //error message     $errorMsg = 'Error on line '.$this->getLine().' in '.$this->getFile()     .': <b>'.$this->getMessage().'</b> is not a valid E-Mail address';     return $errorMsg;     }   } $email = "someone@example...com"; try   {   //check if   if(filter_var($email, FILTER_VALIDATE_EMAIL) === FALSE)     {     //throw exception if email is not valid     throw new customException($email);     }   } catch (customException $e)   {   //display custom message   echo $e->errorMessage();   } ?>

The new class is a copy of the old exception class with an addition of the errorMessage() function. Since it is a copy of the old class, and it inherits the properties and methods from the old class, we can use the exception class methods like getLine() and getFile() and getMessage().

Example explained:

The code above throws an exception and catches it with a custom exception class:

1.     The customException() class is created as an extension of the old exception class. This way it inherits all methods and properties from the old exception class

2.     The errorMessage() function is created. This function returns an error message if an e-mail address is invalid

3.     The $email variable is set to a string that is not a valid e-mail address

4.     The "try" block is executed and an exception is thrown since the e-mail address is invalid

5.     The "catch" block catches the exception and displays the error message

Multiple Exceptions

It is possible for a script to use multiple exceptions to check for multiple conditions.

It is possible to use several if..else blocks, a switch, or nest multiple exceptions. These exceptions can use different exception classes and return different error messages:

<?php class customException extends Exception { public function errorMessage() { //error message $errorMsg = 'Error on line '.$this->getLine().' in '.$this->getFile() .': <b>'.$this->getMessage().'</b> is not a valid E-Mail address'; return $errorMsg; } } $email = "someone@example.com"; try   {   //check if   if(filter_var($email, FILTER_VALIDATE_EMAIL) === FALSE)     {     //throw exception if email is not valid     throw new customException($email);     }   //check for "example" in mail address   if(strpos($email, "example") !== FALSE)     {     throw new Exception("$email is an example e-mail");     }   } catch (customException $e)   {   echo $e->errorMessage();   } catch(Exception $e)   {   echo $e->getMessage();   } ?>

Example explained:

The code above tests two conditions and throws an exception if any of the conditions are not met:

1.     The customException() class is created as an extension of the old exception class. This way it inherits all methods and properties from the old exception class

2.     The errorMessage() function is created. This function returns an error message if an e-mail address is invalid

3.     The $email variable is set to a string that is a valid e-mail address, but contains the string "example"

4.     The "try" block is executed and an exception is not thrown on the first condition

5.     The second condition triggers an exception since the e-mail contains the string "example"

6.     The "catch" block catches the exception and displays the correct error message

If there was no customException catch, only the base exception catch, the exception would be handled there

Re-throwing Exceptions

Sometimes, when an exception is thrown, you may wish to handle it differently than the standard way. It is possible to throw an exception a second time within a "catch" block.

A script should hide system errors from users. System errors may be important for the coder, but is of no interest to the user. To make things easier for the user you can re-throw the exception with a user friendly message:

<?php class customException extends Exception   {   public function errorMessage()     {     //error message     $errorMsg = $this->getMessage().' is not a valid E-Mail address.';     return $errorMsg;     }   } $email = "someone@example.com"; try   {   try     {     //check for "example" in mail address     if(strpos($email, "example") !== FALSE)       {       //throw exception if email is not valid       throw new Exception($email);       }     }   catch(Exception $e)     {     //re-throw exception     throw new customException($email);     }   } catch (customException $e)   {   //display custom message   echo $e->errorMessage();   } ?>

Example explained:

The code above tests if the email-address contains the string "example" in it, if it does, the exception is re-thrown:

1.     The customException() class is created as an extension of the old exception class. This way it inherits all methods and properties from the old exception class

2.     The errorMessage() function is created. This function returns an error message if an e-mail address is invalid

3.     The $email variable is set to a string that is a valid e-mail address, but contains the string "example"

4.     The "try" block contains another "try" block to make it possible to re-throw the exception

5.     The exception is triggered since the e-mail contains the string "example"

6.     The "catch" block catches the exception and re-throws a "customException"

7.     The "customException" is caught and displays an error message

If the exception is not caught in its current "try" block, it will search for a catch block on "higher levels".

Set a Top Level Exception Handler

The set_exception_handler() function sets a user-defined function to handle all uncaught exceptions.

<?php function myException($exception) { echo "<b>Exception:</b> " , $exception->getMessage(); } set_exception_handler('myException'); throw new Exception('Uncaught Exception occurred'); ?>

The output of the code above should be something like this:

Exception: Uncaught Exception occurred

In the code above there was no "catch" block. Instead, the top level exception handler triggered. This function should be used to catch uncaught exceptions.

Rules for exceptions

• Code may be surrounded in a try block, to help catch potential exceptions
• Each try block or "throw" must have at least one corresponding catch block
• Multiple catch blocks can be used to catch different classes of exceptions
• Exceptions can be thrown (or re-thrown) in a catch block within a try block

A simple rule: If you throw something, you have to catch it.