PHP counting the # of word occurance in a string

$string = 'hi there [hello]this is my hello message[/hello][hello]this is another hello message[/hello]';

// Count the number of occurences:
$count = preg_match_all( '/\\[hello\\]/', $string, $dummy );

// Delete [hello] if not closed
// First replace closed matches, then delete remaining, then replace back

$string = preg_replace( '/\\[hello\\](.*)\\[\\/hello\\]/U', '[qwerty]$1[/qwerty]', $string );

$string = str_replace( '[hello]', '', $string );
$string = str_replace( 'qwerty', 'hello', $string );

echo "Count is: $count
$string";
?>