This is a part of the Hack array proposal

Indexing for Hack arrays will have stricter semantics when using square brackets to match the behavior of Hack Collections.

No Coercion of Keys

When indexing or setting a key of an array, PHP sometimes coerces a value to either an int or string. Hack arrays should not do this. Indexing a Hack array with an invalid key type results in an exception. This also means int-like strings will not be converted to ints.

// These will result in exceptions for Hack arrays
$harray[0.0];
$harray[false];

// The string '0' will not be converted to the int 0
$harray['0'];

Throw on Non-existent Keys

Accessing an non-existent key for a PHP array results in raising a E_NOTICE and returning null. Hack arrays will throw an exception if you attempt to access an element that is not present.

Safe-Indexing

If you do not want to risk an exception being thrown when indexing into a Hack array then you can index using curly braces. Currently indexing with curly braces is the same as indexing with square brackets for PHP arrays. We will slightly change this behavior to return null when the key is not set, but NOT raise a E_NOTICE.

$dict = dict[];
$x = $dict{0}; // $x will be null
$y = $dict[0]; // Exception thrown since 0 is not set

This is meant to emulate the get methods that the various Collection APIs provide.