What is the significance of the notice "Undefined variable: _REQUEST" in PHP when trying to access the $_REQUEST array?

The notice "Undefined variable: _REQUEST" in PHP indicates that the $_REQUEST array is being accessed without being defined or initialized. This issue can be solved by checking if the $_REQUEST array is set before accessing its elements to avoid the notice.

if(isset($_REQUEST['key'])){
    // Access $_REQUEST['key'] safely here
}