Variable functions

As you have seen already, PHP has variable variables so it is not surprising we have variable functions. This particular piece of clever functionality allows you to write code like this:

<?php
    $func = "sqrt";
    print $func(49);
?>

PHP sees that you are calling a function using a variable, looks up the value of the variable, then calls the matching function. The code above will therefore return 7 - the square root of 49.

As variable functions are quite unusual and also easy to get wrong, there is a special PHP function, is_callable(), that takes a string as its only parameter and returns true if that string contains a function name that can be called using a variable function. Thus, our script becomes this:

<?php
    $func = "sqrt";
    if (is_callable($func)) {
        print $func(49);
    }
?>

As an alternative to variable functions, you can use call_user_func() and call_user_func_array(), which take the function to call as their first parameter. The difference between the two is that call_user_func() takes the parameters to pass into the variable function as multiple parameters to itself, whereas call_user_func_array() takes an array of parameters as its second parameter.

This next script demonstrates both of these two performing a functionally similar operation, replacing "monkeys" with "giraffes" in a sentence using str_replace():

<?php
    $func = "str_replace";
    $output_single = call_user_func($func, "monkeys", "giraffes", "Hundreds and thousands of monkeys\n");
    $params = array("monkeys", "giraffes", "Hundreds and thousands of monkeys\n");
    $output_array = call_user_func_array($func, $params);
    echo $output_single;
    echo $output_array;
?>

Although call_user_func() is essentially the same as using a variable function, call_user_func_array() is very helpful for functions that have complex and variable parameter requirements. One popular application for variable functions is to allow other developers using your code to register callbacks - they pass in the name of the function they want your code to call, then you can use call_user_func() to execute that.

Copyright ©2015 Paul Hudson. Follow me: @twostraws.