The answer is that those messages are being logged, but to a different log file than where the error_log()
messages go. In my Apache conf, each virtual host has its own error log file. There is also a global error log file where Apache startup/shutdown messages show up.
I was running these LDAP debug scripts on a virtual host and expecting the error output to go to the host's defined error log... just like all other error messages do. But for some reason those ldap debug lines get sent to the global error log.