How to Fix "Call to undefined function" Error

Table Of Contents

• Problem
• Solution
• Explanation

Problem

You get a "Fatal error: Uncaught Error: Call to undefined function" when calling a function that PHP cannot find or access.

Solution

// 1. Check function name spelling
// Wrong:
strlen_count($string); // Fatal error

// Correct:
$length = strlen($string);

// 2. Check if function exists before calling
if (function_exists('imagecreatetruecolor')) {
    $image = imagecreatetruecolor(800, 600);
} else {
    die('GD extension is not installed');
}

// 3. Enable required PHP extensions
// Check if extension is loaded
if (!extension_loaded('curl')) {
    die('cURL extension is required');
}

// Use curl functions
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'https://api.example.com');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$response = curl_exec($ch);
curl_close($ch);

// 4. Include required files for custom functions
require_once 'helpers.php';

function myCustomFunction() {
    return "Hello World";
}

// Now you can use the function
echo myCustomFunction();

// 5. Use autoloading for classes and functions
// composer.json autoload section:
/*
{
    "autoload": {
        "files": ["src/helpers.php"],
        "psr-4": {
            "App\\": "src/"
        }
    }
}
*/

// 6. Check namespaced functions
namespace App\Utils;

function calculateTotal($items) {
    return array_sum($items);
}

// Use with full namespace
$total = \App\Utils\calculateTotal([10, 20, 30]);

// Or with use statement
use function App\Utils\calculateTotal;
$total = calculateTotal([10, 20, 30]);

// 7. Common extension functions and their requirements
$functions_and_extensions = [
    'curl_init' => 'curl',
    'json_encode' => 'json',
    'mysqli_connect' => 'mysqli',
    'imagecreate' => 'gd',
    'openssl_encrypt' => 'openssl',
    'mb_strlen' => 'mbstring'
];

foreach ($functions_and_extensions as $function => $extension) {
    if (!function_exists($function)) {
        echo "Install {$extension} extension for {$function}()\n";
    }
}

// 8. Debugging function availability
$all_functions = get_defined_functions();
if (in_array('my_function', $all_functions['user'])) {
    echo "Function exists in user-defined functions";
}

Explanation

This error occurs when PHP cannot locate a function. Check spelling, ensure required extensions are installed, include necessary files, and verify namespaces are correct.

Use function_exists() and extension_loaded() to check availability before calling functions. Enable missing extensions in php.ini or install them via package manager.