In PHP, how do I check if a function exists?

Question

How can I check if the function my_function already exists in PHP?

Answer 1

And if my_function is in namespace:

namespace MyProject;

function my_function() {
    return 123;
}

You can check if it exists

function_exists( __NAMESPACE__ . '\my_function' );

in the same namespace or

function_exists( '\MyProject\my_function' );

outside of the namespace.

P.S. I know this is a very old question and PHP documentation improved a lot since then, but I believe people still taking a peek here, and this might be helpful.