1.) an double click select an whole "word"
2.) an word is something what is enclosed from an space ( " word " ) OR an "non-word"-sign ( "$word " ) before and after
3.) what belongs to the group of valid word signs is defined in the syntax files in the section <Words>...</Words>
But in the PHP syntax file there is no such section, because:
4.) the PHP syntax file "php_spec.xml" is based on "defsource_spec.xml" and this on "def_spec.xml"
5.) in this "def_spec.xml" you will find this section: <Words>0-9A-Za-z_</Words>
Modify this section to include the '$' -sign as valid sign for an "word" like >>> <Words>0-
9$A-Za-z_</Words>
Then you can select an word like "$myVar" by an double click too.
some notes:
- Modification in this basic syntax file "def_spec.xml" belongs to all syntaxes based on this base.
- You can modify the "php_spec.xml" too to overwrite the settings from "def_spec.xml"... but that is an other story
and can be found in the FAQ section too, ... i would have to take an look myself too.
- I have no glue if this modification will have any side-effects, take care yourself and report how it works for you.
HTH?

Happy
HippoEDITing