OK, I thought that I should put my case here .... I was getting the post array empty in specific cases .. The form works well, but some times users complain that they hit submit button, and nothing happens ..... After digging for a while, I discovered that my hosting company has a security module that checks users inputs and clears the whole post array (not only the malicious data) if it discovers so. In my example, a math teacher was trying to enter the equation: dy + dx + 0 = 0; and data was wiped completely.
To fix this, I just advise him now to enter the data in the text area as dy + dx + 0 = zero, and now it works .... This can save someone some time ..