file_exists () works fine with reletive paths, but seems to not work with absolute paths, or paths starting with '../'

this works fine:
--------------------------------------------------------------
if(file_exists(test/123)) {
            die ("error message");
    }


this does not work:
--------------------------------------------------------------
if(file_exists(/tmp/test/123)) {
            die ("error message");
    }



this does not work:
--------------------------------------------------------------
if(file_exists(../../tmp/test/123)) {
            die ("error message");
    }


thanks... atom