Если не multibyte:
$str=trim($str,"\t\n\r\0\x0B\s");
$str=$str[1];
Если мультибайт:
$re = '/\S{1}/su';
$str = ' But I must explain to you how all this mistaken';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($matches);