Convert a column of values from a 2d array into a delimited string

Array ( 
    [0] => Array ( 
        [0] => 21 
        [SLOTID] => 21
    ) 
    [1] => Array ( 
        [0] => 4 
        [SLOTID] => 4 
    ) 
)

I want to create a new string from this array like:21,4.

$str = $arr[0][0] . ',' . $arr[1][0];