Direto utilizando PHP

$servername = 'localhost';
$username = 'username';
$password = 'password';
$dbname = 'moodle';

$u_moodle = 'theusernameyouwant';
$hp_moodle = password_hash('thepasswordyouwant', PASSWORD_DEFAULT); ///IMPORTANT!
$name = 'first name';
$lname = 'last name';
$email = 'e@m.ail'; ///This have to be verified by you as we're inserting it directly
$course = '123'; //Id that you put in moodle admin, not the real id

$conn = new mysqli($servername, $username, $password, $dbname);

$sql = "INSERT INTO 'mdl_user' (auth, confirmed, mnethostid, username, password, firstname, lastname, email)
VALUES ('manual', 1, 1, '$u_moodle', '$hp_moodle', '$name', '$lname', '$email')";
// auth = 'manual', confirmed = 1, mnethostid = 1 Always. the others are your variables

if ($conn->query($sql) === TRUE) {
echo "OKTC";
} else {
////Manage your errors
}

$sql = "SELECT * FROM $m_user WHERE email='$email'";
$result = $conn2->query($sql);
if($row = $result->fetch_assoc()) {
$id = $row['id']; //Id of newly created user. we're using that for to register him on the course
}

////You have to use this if your idnumber for the course is the one you put into moodle (thats not the real id)
$sql = "SELECT id FROM 'mdl_course' WHERE idnumber=$course";
$result = $conn->query($sql);
if(!$result){
///Not existing course, manage your error
}
if($row = $result->fetch_assoc()) {
$idcourse = $row["id"];
}

///I need now the "enrol" id, so I do this:
$sql = "SELECT id FROM 'mdl_enrol' WHERE courseid=$idcourse AND enrol='manual'";
$result = $conn->query($sql);
if(!$result){
///Not enrol associated (this shouldn't happen and means you have an error in your moodle database)
}
if($row = $result->fetch_assoc()) {
$idenrol = $row["id"];
}

///Lastly I need the context
$sql = "SELECT id FROM 'mdl_context' WHERE contextlevel=50 AND instanceid=$idcourse"; ///contextlevel = 50 means course in moodle
$result = $conn->query($sql);
if(!$result){
///Again, weird error, shouldnt happen to you
}
if($row = $result->fetch_assoc()) {
$idcontext = $row["id"];
}

///We were just getting variables from moodle. Here is were the enrolment begins:

$time = time();
$ntime = $time + 60*60*24*$duration; //How long will it last enroled $duration = days, this can be 0 for unlimited.
$sql = "INSERT INTO 'mdl_user_enrolments' (status, enrolid, userid, timestart, timeend, timecreated, timemodified)
VALUES (0, $idenrol, $id, '$time', '$ntime', '$time', '$time')";
if ($conn->query($sql) === TRUE) {
} else {
///Manage your sql error
}

$sql = "INSERT INTO 'mdl_role_assignments' (roleid, contextid, userid, timemodified)
VALUES (5, $idcontext, '$id', '$time')"; //Roleid = 5, means student.
if ($conn->query($sql) === TRUE) {
} else {
//manage your errors
}

Utilizando via web services

// First get the token.
$tokenurl = 'http://www.yourmoodlesite.com/login/token.php?username=testuser&password=xx&service=myservice';

$tokenresponse = file_get_contents($tokenurl);

$tokenobject = json_decode($tokenresponse);

if (!empty($tokenobject->error)) {
echo $tokenobject->error;
die();
}

// Then call the create user and enrol functions
// Remember to add the question mark after "server.php" because http_build_query() won't add it on its own and you'll end up with a 404 error
$baseurl = 'http://www.yourmoodlesite.com/webservice/rest/server.php?';

// Then add these parameters to the url.

$users = array();
// See the api documentation /admin/webservice/documentation.php
// for core_user_create_users for building the $users array
// e.g.
// $users = array(array(
// 'username' => 'lecapitaine', //Username policy is defined in Moodle security config
// 'password' => 'EngageNCC-1701', //Plain text password consisting of any characters
// 'firstname' => 'William', //The first name(s) of the user
// 'lastname' => 'Shatner', //The family name of the user
// 'email' => 'jimmy.k@enterprise.com',
// 'lang' => 'en',
// ));

$params = array(
'wstoken' => $tokenobject->token,
'wsfunction' => 'core_user_create_users',
'moodlewsrestformat' => 'json',
'users' => $users,
);

$url = $baseurl . http_build_query($params);

$response = file_get_contents($url);

$newusers = json_decode($response);

// Newusers will be an array of objects containing the new user ids.

$enrolments = array();
// See the api documentation /admin/webservice/documentation.php
// for enrol_manual_enrol_users for building the $enrolments array

// Then enrol the users.
$params = array(
'wstoken' => $tokenobject->token,
'wsfunction' => 'enrol_manual_enrol_users',
'moodlewsrestformat' => 'json',
'enrolments' => $enrolments,
);

$url = $baseurl . http_build_query($params);

$response = file_get_contents($url);

$enrolled = json_decode($response);