$re = '/(\/page\/)\d+/';
$str = 'http://retail.domain.co.uk/view_orders/page/6/10?type=completed';
$subst = "$11";
$result = preg_replace($re, $subst, $str, 1);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php