realpath関数の相対パス取得版

function relativepath($dst) {
    switch (false) {
        case $src = getcwd():
        case $dst = realpath($dst):
        case $src = explode(DIRECTORY_SEPARATOR, $src):
        case $dst = explode(DIRECTORY_SEPARATOR, $dst):
        case $src[0] === $dst[0]:
            return false;
    }
    $cmp =
        DIRECTORY_SEPARATOR === '\\' ?
        'strcasecmp' :
        'strcmp'
    ;
    for (
        $i = 0;
        isset($src[$i], $dst[$i]) && !$cmp($src[$i], $dst[$i]); 
        ++$i
    );
    return implode(
        DIRECTORY_SEPARATOR,
        array_merge(
            array('.'),
            ($count = count($src) - $i) ?
                array_fill(0, $count, '..') :
                array()
            ,
            array_slice($dst, $i)
        )
    );
}