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If I want to use a PHP non-associative array like a dictionary and add a big key, how much memory will PHP allocate? 

$myArray = Array();
$myArray[6000] = "string linked to ID 6000";
$myArray[7891] = "another key-value pair";

Will PHP also allocate memory for the unused keys 0-5999 and 6001-7890?

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No, PHP doesn't implement this like a C style array. Php arrays are associative containers, as the php article on arrays states.

An array in PHP is actually an ordered map. A map is a type that associates values to keys.

Since order is preserved, the array will likely be some kind of binary search tree. If you're unfamiliar with binary search trees I suggest picking up a good data structures book to learn more or check out this wikipedia article for a rundown. Your example above would yield a binary search tree with two nodes -- one for data at key 6000, the other for key 7891.

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    Note that if you pass this through json_encode(), that'll create the 0-5999 keys - json doesn't like PHP's sparse arrays.
    – Marc B
    Commented Mar 5, 2011 at 16:08
  • When I do $myArray[strtotime($date)] = ... this runs of of memory, which seems to suggest otherwise.
    – William Entriken
    Commented May 9, 2016 at 16:05
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    No, whupps. I was trying to index a string with a huge number. That WILL crash. Stupid dynamic typing.
    – William Entriken
    Commented May 9, 2016 at 17:56
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It won't allocate memory for indexes 0-5999.

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