I would like to lock row for update but it can't lock .If I lock row in sql*plus by sql command "select * from table where ... for update nowait" it can check lock and display massage 'read only'. How can I do?

my code:
...
    $gcom ='select * from admin.USERS where ACCOUNT = '."'".$ACCOUNT."' for update nowait";
    $result = OCIParse($conn, $gcom);
    $chk = OCIExecute($result,OCI_DEFAULT); 
   if(!$chk){
     $gcom ='select * from admin.USERS where ACCOUNT = '."'".$ACCOUNT."'";
     $mess = "Read only";
    $result = OCIParse($conn, $gcom);
    $chk = OCIExecute($result,OCI_DEFAULT); 
    }else {$mess="Read/Write";}
    echo $mess;
     $rows = OCIFetchStatement($result,$results);
     ...