PHP function_exists() Function: Usage and Examples

Use function_exists() when you need to checks whether a function has been defined and is callable by name in a PHP application. The examples below show the actual operation and its limitations.

What Is function_exists() in PHP?

Checks whether a function has been defined and is callable by name. The exact signature supported by your installation depends on its PHP version and enabled extensions.

Its syntax is:

function_exists(string $function): bool

Parameters and return value: The parameter types, optional arguments, references, and return type are shown in the signature above. Optional parameters are marked with an ellipsis in this reference signature.

Example 1: Basic Usage

<?php

var_dump(function_exists("strlen"));

Output:

bool(true)

Example 2: Different Input or Behavior

<?php

var_dump(function_exists("not_a_real_function_xyz"));

Output:

bool(false)

Practical Example: Applying the Function

<?php

$optionalFeatures = ["mb_strlen", "iconv_strlen"]; foreach ($optionalFeatures as $feature) echo $feature . ": " . (function_exists($feature) ? "available" : "missing") . PHP_EOL;

Output:

mb_strlen: missing
iconv_strlen: available

Important Behavior and Edge Cases

It does not prove that an extension is enabled for a particular operation or that a call will succeed.

Comparison With Related Functions

Common Beginner Mistakes

Key Takeaways

• Checks whether a function has been defined and is callable by name.
• It does not prove that an extension is enabled for a particular operation or that a call will succeed.
• is_callable() also checks callable syntax and methods; function_exists() checks functions by name.

Use function_exists() for the specific operation demonstrated above. Check its prerequisites and handle failure conditions before using it with production data.