So, if I understand correctly, to have pureply numerical Captcha ouput, $t_symbols needs to be set to "2345678" (omitting 0, 1 and 9 to save confusion with letters o, O, l, and g), correct?
And $symbAmount sets the length of the output.
Users do not related to $secretTuringPass at all. It is used in the coding process only, the image itself could be displayed only using
tag, images can not be embedded in HTML directly. That's why while referring to the image containing a certain code, PHP generates some encoded string for this and uses sophisticated protection password to not allow this string easily decoded with brute-force method and others.