php如何把一个二维数组转换成一个一维数组

$arr=array(
	'0'=>array(
		'a'=>2,
		'b'=>'fd',
		'c'=>'fgd',
		'd'=>'hg',
	),
	'1'=>array(
		'e'=>2,
		'b'=>'fd',
		'f'=>'fgd',
	),
	'2'=>array(
		'a'=>2,
		'c'=>'fd',
		'e'=>'fgd',
		'b'=>'hg',
	),
	'3'=>array(
		'c'=>2,
		'd'=>'fd',
		'e'=>'fgd',
		'f'=>'hg',
	),
);
foreach($arr as $k=>$v){
	foreach($v as $key=>$val){
		$arr2[$key]=$val;
	}
}