Determine if a function exists

When we create custom functions and understand the use of mutable functions, in order to ensure that the functions called by the program are present, we often use function_exists to determine whether a function exists. The same method_exists can be used to detect whether a class's methods exist.

Determine if a function exists  function_exists ()
function  func () {

}

if (function_exists(' func '))

echo"presence"

Determine if a class exists class_exists ()
Class  func{

Checks if     the class exists before using the IF (class_exists(' func '))    {           $func = new Func ();            }   

Determine if a file exists

$filename = ' test.txt '; if (! file_exists ($filename)) {    echo$filename . ' Not exists. ' ;}

Determine if a function exists