Will php's json_encode() always use double quotes as string delimiter?

Yes, as defined in the JSON spec, the delimiter will always be ". However, values may contain ' characters, which would break your HTML. To keep it simple and not worry about what might or mightn't pose an issue, HTML-escape your values!

<section data-settings="<?= htmlspecialchars(json_encode($foo)); ?>"></section>

This is guaranteed to work, always, no matter what values you pipe in or how you encode them.

NOTE that htmlspecialchars will by default only encode ", not '; so you must use " as the delimiter in HTML (or change the default escaping behavior).