Just for curiosity, I typed `delete` on an editor with PHP syntax highlighting and it put it with a known keyword color.  

There's `unset()` which when used on a reference I assume triggers garbage collection, resource freeing or whatever PHP does under the hood, but what does `delete` do?  

When trying `delete $ref` on a reference variable, I get this error: `Parse error: syntax error, unexpected T_VARIABLE in /... on line ...`