You may download my php example here.

Here you will find instructions to create Web Services on Moodle.
I am not a Moodle expert. I have collected these information from many different sources and I have eliminated their errors to give you an easy to follow tutorial together with a working example.
This tutorial creates a web service, enabled to create a new user and to enroll this user to a course.

Please follow the steps below:

1) Create the user which will serve the web services.
Go to:
Site Administrator > Users > Accounts > Add new user
Username: wsuser (user your own)
Password: (user your own)
Fill in the required fields.

2) Give the wsuser the appropriate role.
Go to:
Site Administrator > Users > Permissions > Define roles
Press the Add new role button.
Select Role archetype = ARCHETYPE: Manager and press continue.
On the form:
Web Service User
Context types where this role may be assigned: System/User/Category/Course
Leave the rest selections as they appear.
Capabilities
Add the following capabilities:
webservice/rest:use
moodle/user:create
enrol/manual:enrol
enrol/category:synchronised
enrol/manual:unenrolself
Press Create this role button.

3) Assign System roles.
Go to:
Site Administrator > Users > Permissions > Assign System Roles
Assign wsuser user.

4) Create the needed services.
Go to:
Site Administrator > Plugins > Web Services > External Services
We need the two services below:

a) Create user – service
On custom services table press add.
Name: create_user
Short: create_user
“Enabled” should be checked.
“Authorized users only” should be checked.
Press “show more”.
Search for capability moodle/user:create and select it.
Press “add service”.
This service has no functions.
Add functions.
Search for the core_user_create_user
Select it and press add function.

b) Enroll user – service
On custom services table press “add”.
Name: enrol_user
Short: enrol_user
“Enabled” should be checked.
“Authorized users only” should be checked.
Press “show more”.
Search for capability enroll/manual:enrol and select it.
Press “add service”.
This service has no functions.
Add functions.
Search for the enrol_manual_enrol_users
Select it and press “add function”.
On the list of custom services you have to add the authorized user to be the wsuser.

5) Create the Tokens.
Go to:
Site Administrator > Plugins > Web Services > External Services > Manage Tokens
Add
a) Create User
Select user Web Service User
Service – create user
Save changes.

b) Enroll User
Select user Web Service User
Service – enrol user
Save changes.

You now have the two required tokens.

6) All web service protocols are disabled. The «Enable web services» setting can be found in Advanced features.
Go to:
• Site Administrator > Advanced features > Enable web services checked – Save.
• Site Administrator > Plugins > Web Services > External Services > Manage Protocols.
Activate REST protocol.
Check the documentation box.

7) CURL.php is required on the client side. You may download it here.
8) Check if create user works using php code.
9) Check if enroll user works using php code.
10) The wsuser user can not assign role without being enrolled in the course/s!!!

Best regards
Pavlos