<?php
function abc($a) {
if($a == 'a') {
 $b = 'stuff';
}
print_r(debug_backtrace());
}

abc('a');
?>

Array
(
    [0] => Array
        (
            [file] => /t.php
            [line] => 9
            [function] => abc
            [args] => Array
                (
                    [0] => a
                )

        )

)