$upper = array( 
			1 => array(
				1 => 'ONE',
				2 => 'TWO',
				3 => 'THREE' ),
			2 => 'TWO',
			3 => 'THREE' );

$lower = array();

foreach ($upper as $b) {
	if ( !is_string($b) ) {
		$lower[] = $b;		
	}
	else {
		$lower[] = strtolower($b);
	}
}
print_r($lower);