You can use json_decode() php function.
$json_string = '[{"id_piesa":"8","cantitate_piesa":"12","garantie_piesa":false},{"id_piesa":"30","cantitate_piesa":1,"garantie_piesa":true}]';
$array = json_decode( $json_string , TRUE);
print_r( $array);
Output:
Array
(
    [0] => Array
        (
            [id_piesa] => 8
            [cantitate_piesa] => 12
            [garantie_piesa] => 
        )
    [1] => Array
        (
            [id_piesa] => 30
            [cantitate_piesa] => 1
            [garantie_piesa] => 1
        )
)
if you add TRUE as second argument the returned objects will be converted into associative arrays.
Here you can find the complete documentation.