Question

how can I keep the p= and add an OR /20 on it ? I mean I want to check both strings.

<?php if ( false==stripos(get_permalink($post->ID), 'p=') ) { ?>

Answer 1

You cannot test for two portions of strings with a single stripos() call : you have to call stripos() twice.

Depending on what you exactly want to achieve (not sure I really undertand the question), you’ll combine those two with && or || :

$link = get_permalink($post->ID);
if (stripos($link, 'p=')!==false && stripos($link, '/20')!==false) {
    // the link contains both p= AND /20
}

or :

$link = get_permalink($post->ID);
if (stripos($link, 'p=')!==false || stripos($link, '/20')!==false) {
    // the link contains p= OR (inclusive) /20
}

As a sidenote : you should use === or !==, as stripos() can return 0 or false — and those don’t have the same meaning.