In PHP, single-quoted strings and double-quoted strings do not have the same features [0]. Double-quoted strings allow variable interpolation, but single-quoted strings do not.
The proposed style rule is to forbid double quotes unless variable interpolation or other escaped characters (like \n character) are used. The goal is to make it clear when we intend to use variable interpolation or other double-quoted strings features. Most of the time, we just want to write a plain string of characters, so those features are not needed and could be unintentionally used.

Please note that this will create a difference with TypeScript where double quotes are mandatory.

[0] https://www.php.net/manual/en/language.types.string.php#language.types.string.syntax.double