The outline in the PHP editor is broken (i.e. outline is empty) when a "<" (less than sign) character is present between the opening/closing PHP tags.
I'm sure I haven't tested all possible scenarios, but the issue definitely presents itself when the "<" is directly followed by one or more a-z/A-Z characters
The following code examples break the PHP outline
Here are the following workarounds that restored the PHP outline
Remove "<" character
Remove closing PHP tag
Separating "<" character and a-z character
Version tested: 22.214.171.124203261335