Undefined function error: function not defined

"Call to undefined function" means your code tried to use a function that doesn't exist or hasn't been loaded yet. This usually happens when: a required file wasn't included, a plugin isn't active, or a PHP extension is missing.

Function file not loaded

If you created a custom function, the file containing it must be loaded before the function is called.

Common mistake:

<?php
echo my_custom_function();  // Function defined below, but not yet loaded!

function my_custom_function() {
  return "Hello";
}
?>

Fix: Define the function before using it, or load it from a separate file:

<?php
require_once('functions.php');  // Load custom functions first
echo my_custom_function();  // Now it's safe to use
?>

WordPress plugin is inactive

If a plugin provides a function and you call it without checking if the plugin is active:

<?php
// This fails if my_plugin is not active
echo my_plugin_function();
?>

Fix: Check if the function exists before calling it:

<?php
if (function_exists('my_plugin_function')) {
  echo my_plugin_function();
} else {
  echo "Plugin not active";
}
?>

PHP extension not installed

Some functions require PHP extensions (like curl_init requires the CURL extension).

Fix: Check which extensions are loaded:

<?php
if (extension_loaded('curl')) {
  echo "CURL is installed";
} else {
  echo "CURL is missing - contact hosting to install it";
}
?>

Or check in cPanel → Select PHP Version → Extensions. Make sure required extensions are enabled.

Troubleshooting checklist

• ☐ Check the error message - it shows exactly which function is undefined
• ☐ Verify the function is spelled correctly (case-sensitive)
• ☐ Check if the file containing the function is being included/required
• ☐ For WordPress: verify the plugin providing the function is active
• ☐ Check if the required PHP extension is enabled
• ☐ Check PHP version requirements - older PHP may not have the function

Related: Fatal PHP errors | WordPress troubleshooting

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