The ldap client does not need to be run as a daemon or service. A LDAP server provides a standards based protocol which you can use to access information stored in a LDAP database so a api call via php or whatever will get you the information you require regardless of the type of LDAP server.
It might be worthwhile to investigate how connections are handled by PHP. What I mean is that if your web app is going to do a high volume of authentication then it might be more effective to keep an connection open rather than creating a new connection for each authentication request.
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