Add Sniff to ensure file only uses PHP native functions or functions defined in the current file

[aaronjorbin]

Is your feature request related to a problem?

In WordPress, there are a handful of files that need to function on their own. A sniff that detects the use of functions besides those available from PHP itself and those declared in the file itself.

Describe the solution you'd like

File 1 which would pass without issue:

<?PHP 

function foo( $arg ){
 $arguments = func_get_args(); // allowed since function is a part of the PHP standard lib
}

function bar( $arg ){
 return foo( $arg ); // allowed since foo is defined in this file
}

File 2 which would raise an issue

<?PHP

function baz( $arg ){
  return foo( $arg ); // foo is defined in a different file and these this would trigger an issue
}

?>

Additional context (optional)

This is discussed in https://core.trac.wordpress.org/ticket/61694 which is a follow up to prevent issues like the one described in https://core.trac.wordpress.org/ticket/61680

• [ ] I intend to create a pull request to implement this feature.