• Core
  Using "parent" inside a class without parent is deprecated, and will throw
  a compile-time error in the future. Currently an error will only be
  generated if/when the parent is accessed at run-time.

Refs:

• https://github.com/php/php-src/blob/42cc58ff7b2fee1c17a00dc77a4873552ffb577f/UPGRADING#L303
• Warn on "parent" without parent at compile-time php/php-src#3404
• Add compile-time warning on invalid parent use php/php-src#3564
• php/php-src@a9e6667 (originally committed as hard compile time error)
• php/php-src@deb44d4 (reverted back to deprecation error)

Loosely related to #808

I've opted to add this sniff to the Classes category as the use of the parent keyword in a class without parent requires examining the class declaration statement, though it can be argued that it should be in the Keywords category as the focus is on the parent keyword.