Are there any specific guidelines in using CAPITAL LETTERS for HTML/PHP tags rather than lowercase? Is it okay (in terms of search engine optimization) to have both capital letters tags and lowercase letter tags?
If it doesn't matter, does someone have a recommendation or tool to go through hundreds of PHP/HTML pages and change all appropriate tags to lowercase or uppercase? Can DreamWeaver or Notepad++ or TextPad do such a thing (without screwing up any other text/source, etc)
...Is it okay (in terms of search engine optimization) to have both capital letters tags and lowercase letter tags?...
It really does not matter, an HTML element is an HTML element regardless of case. However you should be consistent in what you use. I personally use lowercase because I'm too lazy to hold SHIFT. As for changing the HTML to one way or another, look at HTML Tidy.
Logic without the fatal effects.
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HTML doesn't care if you use <TABLE> <table> or <TaBlE> since it is not case sensitive.
XHTML is case sensitive and requires all tags tro be in lowercase.
In the early days people made their HTML tags all uppercase to make them stand out from the content. More recently most people use all lowercase since it saves holding the shift key (and if they are using XHTML rather than HTML then the tags must be in lowercase anyway).
I think Search Engines care more about what's in between the tags than their case, unless the doctype is XHTML.
Doctype being XHTML wouldn't really have a profound effect on search engines ( 99% of "xhtml" is sent as text/html ), but a Content-Type would if the uppercase resulted in a yellow screen of death.