For various reasons, the LDAP driver in simpleSAMLphp doesn't have referrals enabled, which causes an LDAP search to fail if your search base isn't the exact tree your users are in, i.e. searching 'dc=example,dc=com'
In the file:
After the ldap_connect() line
If this has been helpful to anyone, let me know and I'll post some more SimpleSAMLphp tutorials