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There’s been a few times when i’ve used PHP’s array_filter() function to filter out some array elements - and I regularly find that I want to pass a parameter to the callback function which will affect the filter results. Unfortunately, this can’t be done with the callback implementation! This is the technique I use to achieve the same effect.

I am suprised that PHP’s implementation of callback functions does not support passing arbitrary parameters, especially since methods of a class / object can be referenced as callback functions by passing an array with the object / class name at index 0 and the method name at index 1.

I’ll normally use this technique when i’m calling array_filter() from within a loop - and I need to filter the array based on some variable which changes on each iteration of the loop. I’d like to pass this variable as a parameter to the callback function, but array_filter() will only pass the current array element to the callback function, so we need to achieve it in another way.

The solution I employ is to create an anonymous function during each loop iteration, using PHP’s create_function(). This creates a function at runtime, from the parameters passed in. We can pass the function definition (i.e. the PHP code) as a string, and use variables, much the same as you would when outputting dynamic variables in HTML output.

Here’s an example:

<?php

// Students and their grade results
$results = array(
 0 => array('name' => 'John Doe',  'grade' => 'A'),
 1 => array('name' => 'Jane Doe',  'grade' => 'A'),
 2 => array('name' => 'Jim Doe',   'grade' => 'B'),
 3 => array('name' => 'Paul Doe',  'grade' => 'C'),
 4 => array('name' => 'Peter Doe', 'grade' => 'C')
);

// We want to know how many strudents achieved each grade.
$grades = array('A','B','C');

foreach($grades as $grade)
{
 // Create anonymous callback function to filter results for this grade.
 $function_body = 'return ( $input[\'grade\'] == "'.$grade.'" );';
 $function_name = create_function('$input', $function_body);
 
 // Filter the results array using the anonymous callback function.
 $filtered_results = array_filter($results, $function_name);
 
 // Write some output.
 echo "students achieving grade $grade: ".count($filtered_results)."<br >\n";
}

?> 

This will output the following:

students achieving grade A: 2
students achieving grade B: 1
students achieving grade C: 2 

There you have it, how to implement an anonymous callback function to be used to filter an array.