Yeah, well if you search the web for “E𝄬 major” I bet you’ll get a lot fewer hits than with “Eb major”. That doesn’t mean that “b” is the correct way to write “flat” and “𝄬” is incorrect, but you use “b” when it’s more convenient.
(𝄬 is apparently the Unicode character for the musical flat sign - my browser doesn’t show it.)
About 8 fonts on my computer us it for zero.
It’s ‘O’ in Leetspeak.
Ø also is used to express dimensional diameters, like in measuring a tube, for example: 2cm inner Ø, 2.5cm outer Ø.
Ø also is used to express dimensional diameters, like in measuring a tube, for example: 2cm inner Ø, 2.5cm outer Ø.
Oops… I think my clone get out of the cage again… 
Sorry, I am correct here. The UPPER CASE Phi has a vertical line, the lower case Phi does not, but has a slash as shown in tomndebb’s post.
Regarding the use of this symbol to represent a diameter of a pipe or tube, I’ve only seen it used to measure outer diameter, with the idea that the symbol is kind of a overwriting of an “O” and a “D”. However, that is merely my anecdotal experience, ad I do not know if it is truly universal.
It actually can represent any diameter - OD, ID, holes, etc.
It’s also used in Geometric Dimensioning and Tolerancing to indicate a circular feature OR to indicate the tolerance is diametrical if it’s used in a feature control frame.
http://www.engineersedge.com/gdt.htm
Sorry, you are incorrect and all you have to do is look at a font chart. Look at the symbol font and you will see the entire greek alphabet, upper and lower case and they are totally different and distinct from the null sign being discussed. The letter o and the number 0 look alike but have different ascii symbols and are not the same thing. The null symbol and the lower case phi do not look alike at all and have different symbols. in fact there are TWO lower case phi in the symbol font (corresponding to f and j) and they both look totally different from the null sign. So, sorry, you are mistaken.
I believe the reason that it is used is that it is part of the basic Latin-1 (ISO 8859-1) Character Set set (#216, Capital O with oblique stroke) whereas you have to change to the symbol font to do the Greek letters which may be messy or impossible depending on what you are doing, but people are still calling it phi (phi = phase, get it?)
A table with the Latin-1 (ISO 8859-1) Character Set (used by browsers) can be found at http://www.pemberley.com/janeinfo/latin1tb.html and, as you can see, the greek letters are not in there but the O-slash is.