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.
End Users are not related to $secretTuringPass at all. It is used in the coding process only, the image itself could be displayed only using <img> 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. $secretTuringPass is just about making the public URL to the image a unique one. Without knowing this value, the actual code on the Captcha image could not be known to decoding machine, if it doesn't know the secret key.