As described here :
http://yaml.org/spec/1.2/spec.html#id2760395

A question mark and space (“? ”) indicate a complex mapping key.

But, when I try to load a valid YAML file using complex mappig, I got an error :

Error parsing YAML, invalid file "diet.fr.yml"

ParseException: Unable to parse at line 3 (near "? "1"").

My file content :

---
diet:
  ? "1"
  :
    name: végétalien

As PECL YAML extension ( https://pecl.php.net/package/yaml ) exists, works fine and seems more complete than symfony's one, wouldn't be more simple to replace Symfony's YAML component by php extension ?

What do you think ?