is_prefix() - checking wheather $A is prefix for $B

["AmirBehzad Eslami" <behzad.eslami@xxxxxxxxx>]

I want to write a function to check
whether string $A is a prefix for string $B or not.

I writing this function in order to prevent directory traversal
during a download request. (e.g., download.php?file=..\index.php)

I want to make sure that the realpath() of the requested file is
within the realpath() of the download-directory. Trying to make
sure that the the $download_dir is a prefix for $filepath.

@see: http://en.wikipedia.org/wiki/Directory_traversal
**
*TWO FUNCTIONS:*

function is_prefix1($prefix, $str) {
    return (0 == strncasecmp($prefix, $str, strlen($prefix)));
}

function is_prefix2($prefix, $str) {
    return (0 === stripos($str, $prefix));
}
*USAGE:*
if (is_prefix1('a', 'abcdef'))
 echo 'prefix1 returned True!', '<br />';

if (is_prefix2('a', 'abcdef'))
 echo 'prefix2 returned True!', '<br />';

------------------------
Do these functions do the same job?
Which one provides better performance?

-behzad