Why array_diff() gives Array to string conversion error? array_diff报错！Array to string conversion err

According to it:

php -r 'array_diff(array("a" => array("b" => 4)), array(1));'
PHP Notice:  Array to string conversion in Command line code on line 1
PHP Stack trace:
PHP   1. {main}() Command line code:0
PHP   2. array_diff() Command line code:1

One of your arrays is multidimensional.

array_diff

 only
checks one dimension of a n-dimensional array. Of course you can check deeper dimensions by using 

array_diff($array1[0],
$array2[0]);

就素说！array_diff这个玩意呢！它只对以为数组管用！嘿嘿！