[PHP] PHP 7.1.0 beta1 特性

PHP 7 都還沒摸熟，PHP7.1 beta就出了...，不努力點還真是追不上技術啊~
來記錄一下這次的features吧!

• Nullable Types
• Square bracket syntax for array destructuring assignment
• Allow specifying keys in list()
• Generalize support of negative string offsets
• Void Return Type
• Class constant visibility modifiers
• Multi catch