如何在数组中搜索重复值

I have multidimensional array :

array (size=4)
    0 => 
        array (size=3)
            'a' => string '' (length=0)
            'b' => string '222.000' (length=8)
            'id' => string '7' (length=1)
    1 => 
        array (size=3)
            'a' => string '61100' (length=0)
            'b' => string '1000.000' (length=8)
            'id' => string '6' (length=1)
    2 => 
        array (size=3)
            'a' => string '61100' (length=5)
            'b' => string '-1000.000' (length=7)
            'id' => string '4' (length=1)

I want to get the value of 'id' of arrays that contain the same 'a' value, in this case i want to get the value of 'id' of arrays 1 & 2 because both having the same value of 'a' so i want to get they id (4 & 6)

Thanks for your replies

You can do it with simple for loop :

$arr = array(["id" => 7, "a" => ""], ["id" => 6, "a" => "AAA"], ["id" => 4, "a" => "AAA"]);

$ans = [];    
foreach($arr as $elem)
        $ans[$elem["a"]][] = $elem["id"];

This will output associative array with the "a" value as keys - if you only want to group them you can use array_values.

Output:

Array
(
    [] => Array
        (
            [0] => 7
        )
    [AAA] => Array
        (
            [0] => 6
            [1] => 4
        )
)