word_pairs.php

<?php
function word_pairs($string, $min_matches = 1) {
	$pair_counts = array();
	$words = explode(' ', $string);
	$previous_word = FALSE;
	foreach($words as $word) {
		if($previous_word)
			$pair_counts[$previous_word.' '.$word]++;
		$previous_word = $word;
	}
	
	$pair_counts = array_filter($pair_counts, function($count) use ($min_matches) {
		return $count >= $min_matches;
	});
	
	return $pair_counts;
}

$string = 'hej hopp hej hopp hej hej hej jag heter johnny vem mer heter johnny';

print_r(word_pairs($string, 2));