I needed to check, whether a directory is allowed through open_basedir..
This should be in the php-functions.

My solution (not tested under windows):

function inOpenBasedir($dir) {
    $openBasedir = ini_get('open_basedir'); 
    if (empty($openBasedir)) { 
        return true; 
    } 

    foreach (explode(':', $openBasedir) as $basedir) 
    { 
	if( strlen($basedir) > strlen($dir) )
	{
	    // Check, if only a '\' is needed at the end of $dir
	    if( $basedir == ($dir . "/") )
	    {
		return true;
	    }
	}
	else
	{
	    // Check if basedir and dir are the same..
	    if( $basedir == $dir )
	    {
		return true;
	    }
	    else
	    {
		// open_basedir can be a prefix -> checking whether
		// dir starts with basedir or not
		if( strncmp($basedir, $dir, strlen($basedir)) == 0)
		{ 
		    return true; 
		} 
				    
	    }
	}	
    }
    return false; 
}