Gedit is a good text editor with many plugins which make it become almost an IDE. In many cases, I prefer it to eclipse for example.
But for php it misses heredoc string highlighting. An heredoc string is for example :

echo <<<
here I can write a big string with some ' or some "
without need to escape them with \ but I can put some {$variables} or {$tableau['0']
HTML;

After a little search, I found the language specs for php here:
/usr/share/gtksourceview-1.0/language-specs/php.lang
So I edited this file (as root, with sudo in my case) and add this block :

<!-- 'heredoc strings' -->
<string _name="Heredoc String" end-at-line-end="FALSE">
<start-regex><<<</start-regex>
<end-regex>[A-Z]+[;]</end-regex>
</string>