<?php
$string = 'The site runs on PHP '.phpversion();
preg_match('/php ((\d)\.\d\.\d+)/i',$string,$matches);
print_r($matches);
vprintf('Match: %s<br /> Version %s; Major:%d.',$matches);
?>

output:
Array ( [0] => PHP 5.2.5 [1] => 5.2.5 [2] => 5 ) 
Match: PHP 5.2.5 Version 5.2.5; Major:5.

For preg_match:

If matches  is provided, then it is filled with the results of search. $matches[0] will contain the text that matched the full pattern, $matches[1]  will have the text that matched the first captured parenthesized subpattern, and so on.