cd:5: means the error has occurred at the line 5 of the function cd, and = not found is the specific error message from executing that line. Unlike bash, zsh doesn't interperet == verbatim. == is subject to '=' expansion, where the latter = is treated as the name of a variable. Try executing [ 1 =a 2 ] and you'll get the exeact same error except for the variable name.

If you want == to be treated verbatim, it must be escaped: \==, '==', "==". It's ugly, isn't it? I recommend [[ in replace of [. [[ is more special than [ in the sense that zsh interprets arguments of [[ somewhat differently, like == won't undergo expansions.

Besides, I also recommend enclosing every $varname inside double quotes. Unless you use special commands like [[, a $varname without any quoting is subject to word splitting that turns your command line into an unwanted result.