How to comment multiple lines in PhpStorm IDE?

There are two possibilities (in cases where block of comment are available like php, json etc files)

First
Windows: Ctrl+/
Mac: ⌘+/

will produce

// comment

Second
Windows: Ctrl+Shift+/
Mac: ⌘+⌥+/

will produce

/* comment */

Select multiple lines and use the Comment with Line Comment (Ctrl+/ in the default keymap on Windows, can be changed in Settings | Keymap).

There is also Comment with Block Comment that works for file types that have special style comment for code blocks (not applicable for .htaccess).