$array[key], $array['key'], $array["key"] ?

[PHP John]

I am wondering just what the differences between the following formats are:

$array[key]

$array['key']

$array["key"]

How does each affect performance?

What are the advantages/disadvantages between each (if there are any)?

[Phil.Roberts]

With:

$array['key']
$array["key"]

They are functionally identical when mere referencing an array key name, but if you were to use a variable:

$array['$key'] : Would pass a literal string '$key'
$array["$key"] : Would pass the value of $key

If you were to use:

$array[key]

Then PHP will first search for a defined constant named "key", if it doesnt find this, then it will regard it as a key name. If you were to set your error reporting to E_ALL you would see the results of using arrays in this fashion, and its to be avoided as it results in a loss of performance.

Similar could be said for $array["$key"], as PHP will expect a string, but will also parse that string for variables. Another performance hit. Just use $array[ $key ].