- Find the loaded configuration file php.ini
- Open php.ini file
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Windows - 
Apache Server - Search for extension=php_ldap.dll in php.ini file. Uncomment or add (if not present) this line and save the file.
 
- 
IIS server - Search for ExtensionList in the php.ini file. Uncomment or add (if not present) the extension=php_ldap.dll line and save the file.
 
 
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- 
Linux - Install php ldap extension (If not installed yet)
For Debian, the installation command would be apt-get install php-ldap For RHEL based systems, the command would be yum install php-ldap
- Search for extension=php_ldap.so in the php.ini file. Uncomment or add (if not present) this line and save the file.
 
- Install php ldap extension (If not installed yet)
- Restart your server service. After that refresh the LDAP/AD plugin configuration page.