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PHP call to undefined function – Fatal Error

Many PHP developers at least once in their career faced the following error:

Fatal error: Uncaught Error: Call to undefined function

To fix it, you should find the reason why the function happens to be undefined.

Here are the most popular ones:

  • You misspelled the function name. E.g. instead of referring to changeDate() you referred to chngeDate(). Make sure the function name is the same as the declared one.
  • You didn’t include the file with the function properly. If the function is located in the external file, make sure you loaded it with include() or require_once().
  • You are referring to a PHP function that is not supported with the PHP version installed on your server.
  • You used the namespaced method without specifying the namespace (e.g. with use).

Example of properly working function:

<?php
function testFunction() {
    return "I'm a function!";
}

echo testFunction();

Output: I’m a function!