Interpolation (double quoted string) of Associative Arrays in PHP
Yes, you may trust it. All ways of interpolation a variable are covered in the documentation pretty well.
If you want to have a reason why this was done so, well, I can't help you there. But as always: PHP is old and has evolved a lot, thus introducing inconsistent syntax.
Yes, this is well defined behavior, and will always look for the string key 'key', and not the value of the (potentially undefined) constant key.
For example, consider the following code:
$arr = array('key' => 'val');define('key', 'defined constant');echo "\$arr[key] within string is: $arr[key]";This will output the following:
$arr[key] within string is: valThat said, it's probably not best practice to write code like this, and instead either use:
$string = "foo {$arr['key']}"or
$string = 'foo ' . $arr['key']syntax.
The last one is a special case handled by the PHP tokenizer. It does not look up if any constant by that name was defined, it always assumes a string literal for compatibility with PHP3 and PHP4.