I have ran into an issue/bug with zend studio/eclipse IDE, every once an a while code completion will stop working. I have not been able to pin down what exactly causes the problem, but here is my fix.

Delete the following file in the Zend Studio workspace:

Zend/workspaces/<your workspace name>/.metadata/.plugins/org.eclipse.core.runtime/.settings/org.eclipse.dltk.ui.prefs