How to reverse words in a string?

$string = "I am a boy";

$reversed = "";
$tmp = "";
for($i = 0; $i < strlen($string); $i++) {
    if($string[$i] == " ") {
        $reversed .= $tmp . " ";
        $tmp = "";
        continue;
    }
    $tmp = $string[$i] . $tmp;    
}
$reversed .= $tmp;

print $reversed . PHP_EOL;
>> I ma a yob

Whoops! Mis-read the question. Here you go (Note that this will split on all non-letter boundaries, not just space. If you want a character not to be split upon, just add it to $wordChars):

function revWords($string) {
    //We need to find word boundries
    $wordChars = 'abcdefghijklmnopqrstuvwxyz';
    $buffer = '';
    $return = '';
    $len = strlen($string);
    $i = 0;
    while ($i < $len) {
        $chr = $string[$i];
        if (($chr & 0xC0) == 0xC0) {
            //UTF8 Characer!
            if (($chr & 0xF0) == 0xF0) {
                //4 Byte Sequence
                $chr .= substr($string, $i + 1, 3);
                $i += 3;
            } elseif (($chr & 0xE0) == 0xE0) {
                //3 Byte Sequence
                $chr .= substr($string, $i + 1, 2);
                $i += 2;
            } else {
                //2 Byte Sequence
                $i++;
                $chr .= $string[$i];
            }
        }
        if (stripos($wordChars, $chr) !== false) {
            $buffer = $chr . $buffer;
        } else {
            $return .= $buffer . $chr;
            $buffer = '';
        }
        $i++;
    }
    return $return . $buffer;
}

Edit: Now it's a single function, and stores the buffer naively in reversed notation.

Edit2: Now handles UTF8 characters (just add "word" characters to the $wordChars string)...