Checking function definition using function_exists

function_exists()
function_exists — Return TRUE if the given function has been defined

Syntax

bool function_exists ( string $function_name )

Example

<!DOCTYPE HTML>
<html>
    <head>
        <title>php function_exists</title>
    </head>
    <body>
    <?php
    
    function testFunction ()
    {
        echo " hello from function";
    }
    
    if (function_exists('testFunction')) 
    {
        echo "function already exists";
        testFunction();
    } 
    else 
    {
        echo "creating testFunction() first time";
    }
    
    ?>
    </body>
</html>

Output

function already exists hello from function