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A callback function (often referred to as just "callback") is a function which is passed as an argument into another function.
Callback Functions
A callback function (often referred to as just "callback") is a function which is passed as an argument into another function.
Any existing function can be used as a callback function. To use a function as a callback function, pass a string containing the name of the function as the argument of another function:
Example
Pass a callback to PHPs array_map() function to calculate the length of every string in an array:
function my_callback($item) {
return strlen($item);
}
$strings = ["apple", "orange", "banana", "coconut"];
$lengths = array_map("my_callback", $strings);
print_r($lengths);
?>Starting with version 7, PHP can pass anonymous functions as callback functions:
Example
Use an anonymous function as a callback for PHPs array_map() function:
$strings = ["apple", "orange", "banana", "coconut"];
$lengths = array_map( function($item) { return strlen($item); } , $strings);
print_r($lengths);
?>Callbacks in User Defined Functions
User-defined functions and methods can also take callback functions as arguments. To use callback functions inside a user-defined function or method, call it by adding parentheses to the variable and pass arguments as with normal functions:
Example
Run a callback from a user-defined function:
function exclaim($str) {
return $str . "! ";
}
function ask($str) {
return $str . "? ";
}
function printFormatted($str, $format) {
// Calling the $format callback function
echo $format($str);
}
// Pass "exclaim" and "ask" as callback functions to printFormatted()
printFormatted("Hello world", "exclaim");
printFormatted("Hello world", "ask");
?>
