Call to Undefined Function/Method

TL;DR

Explanation

This fatal error happens when: (1) calling a function that doesn't exist — typo or not yet defined, (2) calling a function from a file not yet loaded — use require/autoload, (3) calling a function from a PHP extension not installed/enabled (e.g. imagecreatetruecolor without ext-gd), (4) calling a method on a wrong type (calling string methods on null — PHP 8 throws TypeError). Extension functions: use extension_loaded('gd') to check. Static method errors: 'Call to undefined method' — check class name, method visibility, and namespace.

Common Misconception

Why It Matters

Common Mistakes

• Not declaring extension dependencies in composer.json require section.
• Calling instance methods statically or vice versa.
• Using functions from optional extensions without checking extension_loaded().

Code Examples

// ext-gd not installed in production
$img = imagecreatetruecolor(800, 600);
// Fatal: Call to undefined function imagecreatetruecolor()

// In composer.json:
// "require": {"ext-gd": "*"}

// Runtime check:
if (!extension_loaded('gd')) {
    throw new \RuntimeException('GD extension required. Install php-gd.');
}
$img = imagecreatetruecolor(800, 600);

References

• ↗ https://www.php.net/manual/en/function.extension-loaded.php

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