PHP7: backward incompatible changes

Changes to the handling of indirect variables, properties, and methods

Indirect access to variables, properties, and methods will now be evaluated strictly in left-to-right order, as opposed to the previous mix of special cases. The table below shows how the order of evaluation has changed. Code that used the old right-to-left evaluation order must be rewritten to explicitly use that evaluation order with curly braces (see the below middle column). This will make the code both forwards compatible with PHP 7.x and backwards compatible with PHP 5.x.

Old and new evaluation of indirect expressions
Expression PHP 5 interpretation PHP 7 interpretation
$$foo[‘bar’][‘baz’] ${$foo[‘bar’][‘baz’]} ($$foo)[‘bar’][‘baz’]
$foo->$bar[‘baz’] $foo->{$bar[‘baz’]} ($foo->$bar)[‘baz’]
$foo->$bar[‘baz’]() $foo->{$bar[‘baz’]}() ($foo->$bar)[‘baz’]()
Foo::$bar[‘baz’]() Foo::{$bar[‘baz’]}() (Foo::$bar)[‘baz’]()

PHP 4 style constructors
PHP 4 style constructors (methods that have the same name as the class they are defined in) are deprecated, and will be removed in the future. PHP 7 will emit E_DEPRECATED if a PHP 4 constructor is the only constructor defined within a class. Classes that implement a __construct() method are unaffected.