for loop problem

Given this array:

$categories = array(4,15,21,83,122);

the following for loop should be replacing the number 15 with the number 0 but instead it's replacing all the numbers with 0.

$n = count($categories);
	
for($i=0; $i<$n; $i++)
{
	if ($categories[$i] = 15)
	{
		$categories[$i] = 0;
	}
}

Code (markup):

I'm at a loss to figure this out.

 

if ($categories[$i] == 15)

PHP:

 

Ah hell!

I can't tell you how long I've stared at that piece of code and that never occurred to me.

Thanks szalinski, now I can move on to my next obvious bug.