If you want to add any extention in dreamweaver, you need to change two configuration files of dreamweaver.
Step 1: <program files>/<adobe>/<adobe dreamweaver cs4>/configuration . In this folder you need to open “Extensions.txt”
Check above screenshot i have added PHTML extension in “PHP Files”.
Step 2: Open <program files>/<adobe>/<adobe dreamweaver cs4>/configuration/DocumentTypes/MMDocumentTypes.xml this file.
Check above screenshot i have added one more extentions phtml in php model list.