Fatal error: Call to undefined function XXX
Happens when you try to call a function that is not defined yet. Common causes include missing extensions and includes, conditional function declaration, function in a function declaration or simple typos.
Example 1 - Conditional Function Declaration
$someCondition = false;
if ($someCondition === true) {
function fn() {
return 1;
}
}
echo fn(); // triggers error
In this case, fn() will never be declared because $someCondition is not true.
Example 2 - Function in Function Declaration
function createFn()
{
function fn() {
return 1;
}
}
echo fn(); // triggers error
In this case, fn will only be declared once createFn() gets called. Note that subsequent calls to createFn() will trigger an error about Redeclaration of an Existing function.
You may also see this for a PHP built-in function. Try searching for the function in the official manual, and check what "extension" (PHP module) it belongs to, and what versions of PHP support it.
In case of a missing extension, install that extension and enable it in php.ini. Refer to the Installation Instructions in the PHP Manual for the extension your function appears in. You may also be able to enable or install the extension using your package manager (e.g. apt in Debian or Ubuntu, yum in Red Hat or CentOS), or a control panel in a shared hosting environment.
If the function was introduced in a newer version of PHP from what you are using, you may find links to alternative implementations in the manual or its comment section. If it has been removed from PHP, look for information about why, as it may no longer be necessary.
In case of missing includes, make sure to include the file declaring the function before calling the function.
In case of typos, fix the typo.
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