If you are a Windows Users: 
This is a common error when you use XAMPP since LDAP is not enabled by default.
You can follow this steps to make sure LDAP works in your XAMPP

• [Your Drive]:\xampp\php\php.ini In this file uncomment the foll. line: extension=php_ldap.dll
• Move the file: libsasl.dll, from [Your Drive]:\xampp\php to [Your Drive]:\xampp\apache\bin
• Restart Apache.
• You can now use functions of the LDAP Module!

Fatal error: Call to undefined function: ldap_conn