During dynamic loading and linkage by PHP, they won 't resolve because of the typing errors - there are no corresponding symbols in the main binary.
PHP might also complain about missing functions in your module. (This shouldn't happen with the sample sources if you didn't modify them.) If the names of external functions you're trying to access from your module are misspelled, they'll remain as "unlinked symbols" in the symbol table. During dynamic loading and linkage by PHP, they won't resolve because of the typing errors - there are no corresponding symbols in the main binary. Look for incorrect declarations in your module file or incorrectly written external references. Note that this problem is specific to dynamic loadable modules; it doesn't occur with static modules. Errors in static modules show up at compile time.