I wrote three methods in a class and one is calling another, but when I call the function outside through the object, it is showing an undefined function error for the second function.

Here's my code:

function resize_image(){

}

function image_resize(){
    $a = resize_image();
}

When I run this, it shows resize_image() as undefined. Here's the error:

Fatal error: Call to undefined function resize_image() in /home/vacayge/public_html/Major/Alpha1/classes/cUserImages.php on line 2090