Eclipse might be overkill for just casual PHP coding. I've been using Eclipse with the PyDev plugin for Python coding, but I know there are PHP plugins out there. I'm not sure what the default text editor is for KDE (Is it kate?), but they should do for most PHP code and more advanced than a lot of the Windows editors out there with built syntax highlighting.
EDIT: Just checked and I'm guessing kedit is the default KDE text editor, so yeah, definitely check out Kate or kwrite. There should be easily installable packages for your distro for both.