If a text is selected, opening search or replace window will put the selection in the "Text to find" field. This is OK if we work on single lines, but if the selection covers multiple lines, the field should be automatically changed to multiline. Don't you think? (:

The multiline search feature alone is a bit confusing to some people as they did not see it in the other editors and found it a bit cumbersome, so we decided to turn it off by default.
I'm not sure if triggering to multiline would make them happier.
On the other hand, I'd consider a fix like below:
if "find text under cursor" is on and multiple line is selected, find and find/replace dialogs should know full version of the text and if you switch to multiline, it should be shown multiline.

Well it's something, but still not "natural" and effective. People using PhpED are programmers who are supposed to be intelligent, so automatic multiline mode when multiple lines were selected is not something they wouldn't understand... they should expect it - I think (: