<?php

$inputWords = mb_split('\s+', 'If you want to have a preformatted block...');
$forbiddenWords = array("want", "have");
 
$filtered = array_diff($inputWords, $forbiddenWords); // removes blacklisted
 
$pairs = array_chunk($filtered, 2); // array of arrays
$pairs = array_map(function($a) { return implode(' ', $a); }, $pairs);
 
print_r($pairs);