Callback Functions

Table of Contents

A callback function, which is often just called \”callback,\” is a function that is passed to another function as an argument.

A callback function can be any function that is already in use. To use a function as a callback function, you must pass the function\’s name as an argument to another function:

Example

Give PHP\’s array map() function a callback to figure out how long each string in an array is:

<!DOCTYPE html>
<html>
<body>

<?php
function my_callback($item) {
return strlen($item);
}

$colors = [\”Red\”, \”Green\”, \”Blue\”, \”Black\”];
$lengths = array_map(\”my_callback\”, $colors);
print_r($lengths);
?>

</body>
</html>

Output

Array ( [0] => 3 [1] => 5 [2] => 4 [3] => 5 )

Example 2

As of PHP version 7, anonymous functions can be passed as callback functions:

<!DOCTYPE html>
<html>
<body>

<?php
$colors = [\”Red\”, \”Orange\”, \”Yellow\”, \”Pink\”];
$lengths = array_map( function($item) { return strlen($item); } , $colors);
print_r($lengths);
?>

</body>
</html>

Output

Array ( [0] => 3 [1] => 6 [2] => 6 [3] => 4 )

User Defined Functions callbacks.

Callback functions can also be passed as arguments to user-defined functions and methods. To use callback functions inside a user-defined function or method, add parentheses around the variable and pass arguments as you would with regular functions:

Example

Use a user-defined function to run a callback:

<!DOCTYPE html>
<html>
<body>

<?php
function mark($var) {
return $var . \”! \”;
}

function ask($var) {
return $var . \”? \”;
}

function wordmark($var, $mark) {
// Calling the $mark callback function
echo $mark($var);
}

// Pass \”mark\” and \”ask\” as callback functions to wordmark()
wordmark(\”Loream ipsum\”, \”mark\”);
wordmark(\”Loream ipsum\”, \”ask\”);
?>

</body>
</html>

Output

$\"Callbacks$

Callback Functions – PHP