Call to undefined function 🐘 PHP

PHP Fatal error: Uncaught Error: Call to undefined function

PHP can’t find the function you called — usually a missing extension, missing include, or namespace issue.

Seen on: PHP Linux

Meaning

If the function is built into a PHP extension (mysqli_connect, curl_init, mb_strlen, imagecreatefromjpeg), the extension isn’t installed/enabled for the PHP version actually running. Otherwise, the file defining it wasn’t loaded or the name/namespace is wrong.

Common causes

  • PHP extension not installed or not enabled (mysqli, curl, mbstring, gd, intl)
  • CLI and web server use different PHP versions/php.ini
  • File with the function not included / autoloaded
  • Function removed in newer PHP (e.g. mysql_connect removed in PHP 7, each() in PHP 8)
  • Calling a namespaced function without use function or full name

⚡ Quick fix

  1. Install/enable the extension: sudo apt install php8.2-curl (match your version) and restart FPM/Apache
  2. Check phpinfo() from the web server, not just php -m on CLI
  3. Replace removed functions (mysql_* → PDO/mysqli)
  4. Check includes/autoload and namespaces

Detailed fix by platform

PHP

  1. Find which extension is missing:
    bash
    php -v && php -m | grep -i -E 'curl|mysqli|mbstring|gd|intl'
sudo apt install php8.2-curl php8.2-mbstring php8.2-mysql
sudo systemctl restart php8.2-fpm

Code examples

Check whether an extension function exists at runtime

php
foreach (['curl_init', 'mb_strlen', 'mysqli_connect', 'imagecreatetruecolor', 'intl_get_error_code'] as $fn) {
    echo str_pad($fn, 24), function_exists($fn) ? "OK\n" : "MISSING\n";
}
echo 'php.ini: ', php_ini_loaded_file(), "\n";

Run it through the web server (not CLI) — CLI and FPM often load different php.ini files.

Replace removed mysql_* functions with PDO

php
// ❌ PHP 7+: Call to undefined function mysql_connect()
$link = mysql_connect('localhost', 'user', 'pass');

// ✅
$pdo = new PDO('mysql:host=localhost;dbname=app;charset=utf8mb4', 'user', 'pass', [
    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
]);

How to diagnose

  1. Function — Built-in (extension) or yours?
  2. Runtime — Which PHP version/ini does the web server use?
  3. Loading — Is the defining file included?

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