how to add item to array by key in PHP

First off, read this. It explains everything about arrays.

Secondly, your code looks fine. Are you sure you are declaring your array properly? Post your exact code - obsfusticating your code by changing names and values does not help - especially since it can easily cover up the error.

That shouldn't give any warning at all, are you sure the problem isn't with value?

You could add the value to the array by key by doing:

$a = array(
    'key' => 'value',
    'key2' => 'value2'
);

etc etc. This is really only viable if you are creating an array with all the keys known a head of time. You could create an empty array and fill it in with values later on if you want to be super complete and do not have the values a head of time like so:

$a = array(
    'key' => '',
    'key2' => ''
);

$a['key']  = 'value';
$a['key2'] = 'value2';