php - add to array if it isn't there already

Question

How do I add elements to an array only if they aren't in there already? I have the following:

$a=array();
// organize the array
foreach($array as $k=>$v){
    foreach($v as $key=>$value){
        if($key=='key'){
        $a[]=$value;
        }
    }
}

print_r($a);

// Output

Array
(
[0] => 1
[1] => 2
[2] => 3
[3] => 1
[4] => 2
[5] => 3
[6] => 4
[7] => 5
[8] => 6

)

Instead, I want $a to consist of the unique values. (I know I can use array_unique to get the desired results but I just want to know)

Answer 1

You should use the PHP function in_array (see http://php.net/manual/en/function.in-array.php).

if (!in_array($value, $array))
{
    $array[] = $value; 
}

This is what the documentation says about in_array:

Returns TRUE if needle is found in the array, FALSE otherwise.