PHP function_exists: Avoid Function Redeclaration

The function_exists() function in PHP checks whether a given function is defined or not.

The function_exists() function works for both built-in and user-defined functions

PHP function_exists Syntax

PHP function_exists takes only one parameter:

function_exists(string $function_name)

$function_name: The name of the function to check. Pass it as a string. It returns boolean value:

• true if the function exists.
• false if it does not.

Here is a quick example:

if (function_exists('my_custom_function')) {
    my_custom_function();
} else {
    echo "Function not found.";
}

The output:

Function not found

When you call function_exists('my_custom_function'), PHP:

1. Looks for the function name in the global list of defined functions.
2. Checks if the function is available (either built-in or user-defined).
3. Returns true if it finds the function.
4. Returns false if the function is not defined yet.

So what happens behind the scenes? Let’s answer this question in the following section.

Behind the Scenes

Here is how it works:

• PHP keeps a registry of all defined functions (built-in + user-defined).
• function_exists() checks that registry.
• It doesn’t trigger autoloaders. So if the function is in a file that hasn’t been included yet, PHP won’t load it automatically.

So in the following example:

function play_laptop() {
    echo "Opening the windows!";
}

if (function_exists('play_laptop')) {
    play_laptop(); // => This runs
}

Output:

Opening the windows!

The play_laptop function runs because it’s already in PHP’s global function list.

Examples of function_exists() in PHP

You can check a function name using any mix of uppercase or lowercase letters, and PHP will still find it.

For example:

function greatFLATcoding() {
    echo "Hello FlatCoding Students!";
}

if (function_exists('GREATFLATCODing')) {
    greatFLATcoding(); // This still runs
}

Output:

Hello FlatCoding Students

You can use function_exists() to check if a function is already defined before defining it:

if (!function_exists('flatcoding_students')) {
    function flatcoding_students() {
        echo "Hello!";
    }
}

In this example, we first check if the flatcoding_students function does not exist, then we define it.

Wrapping Up

In this tutorial, you covered how the function_exists() function works in PHP and why it’s useful.

Here is a quick recap:

• function_exists() checks if a function is already defined.
• It works for both built-in and user-defined functions.
• It takes one string parameter—the function name—and returns true or false.
• PHP looks in its global list of defined functions to check for existence.
• It doesn’t load functions from files that haven’t been included yet.
• Function names are case-insensitive when using function_exists().

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