Remove $this from incompatible context

[nikic]

• If we can detect at compile-time that a function uses $this,
  then ZEND_ACC_ALLOW_STATIC flag will not be set.
• A static call (either without object or object from an incompatible
  class) to a non-static method without ZEND_ACC_ALLOW_STATIC will
  generate a fatal error.
• If static calls are allowed an E_DEPRECATED will be generated.
• In this case $this will be NULL. This makes sure that even
  obscured (varvar) usages of $this will not be able to access an
  object from an incompatible context.

[mbeccati]

PEAR and other classes historically allowed some methods to be called both ways and $this was used to check whether or not the method was called statically and adjust the behaviour accordingly, e.g. https://github.com/pear/pear-core/blob/stable/PEAR.php#L652

Adding the static modifier removes $this altogether, whereas this PR triggers a fatal error. Using the varvar trick allows the code to work, raising an E_DEPRECATED (vs E_STRICT in 5.6).

[nikic]

Closing as this won't be merged in this form.

[cweiske]

We've already fixed most of this cases in pear-core branch "incompatible context" - https://github.com/pear/pear-core/tree/incompatible-context

So this would not be a problem for us at all.