PHP - check if file existes on parent directory (from root) is impossible?

I was wondering why I can't check if file exists on parent directory (not from root and not from upper directories)

if(file_exists('../1.txt'))
{
    echo 'File Exists!';
}
else
{
    echo 'File Is Not Exists!';
}

Or is it impossible to check if file exists on parent directory ?