php - unexpected T_FUNCTION error

I am using this function

    usort($arr, function($a, $b) use ($str) {return (strpos($a, $str) - strpos($b, $str));});

to sort the array $arr. This function works fine on localhost however when I uploaded my site online this error comes up

Parse error: syntax error, unexpected T_FUNCTION

Does anyone know why this is happening?

Answer

seems like your PHP version on your host is < 5.3.0
and its not support anonymous functions

http://php.net/manual/en/functions.anonymous.php

you may try this

function cmp($a, $b, $str) {

    return (strpos($a, $str) - strpos($b, $str));
}

usort($arr, create_function('$a, $b', 'return cmp($a, $b, "' . $str . '");'));

or upgrade you php on server.