PHP implode Invalid arguments passed

Question

I am using Codeigniter and its validation rules - a custom callback validation. I think this is not  CI-related. 

I've got this function to return a string :

function array_implode($a)
{
  return implode(',', $a);
}

But it always return a message, implode(): Invalid arguments passed

But var_dump() returns this:

array(2) {
  [0]=> string(10) "First item"
  [1]=> string(11) "Second item"
}

What am I doing wrong here? Can someone please help me with this?