create another muti dimentional array from exsisting array and sort a muti dimentional array

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hi, i have question about php arrays. i have array like $data.

	$data=array(
array('a'='1','b'='2','c'='3','d'='4'),                                             array('a'='5','b'='6','c'='7','d'='8'),                        array('a'='9','b'='10','c'='11','d'='12')			
			);

i want to get only a,c,d element and create another multi dimensional array like $data1.

$data1=array(
        array('a'='1','c'='3','d'='4'),
                    array('a'='5','c'='7','d'='8'),
                    array('a'='9','c'='11','d'='12')
        );

as a next step i wanted to sort $data1 array by first value of d elements then by value of c elements and finally by a elements and get array like $data2.

$data2=array(
        array('a'='9','c'='11','d'='12')
        array('a'='5','c'='7','d'='8'),
        array('a'='1','c'='3','d'='4')
        );

i need little bit explained answer to each step. i'm stuck in here this problem. i want help..
and are there any php array function available for get $data1 array from $data easily?