<?php
$string = 'A/B/PA ID U/PA ID U/C/D';
// get only first value starting with PA:
echo preg_match('~PA[^/]+~', $string, $match) ? $match[0] : '';
echo "\n---\n";
// get all values strings with PA:
var_export(preg_match_all('~PA[^/]+~', $string, $matches) ? $matches[0] : []);
echo "\n---\n";
// get all delimited values:
var_export(explode('/', $string));