call a function in different namespacs php

I learn MVC recently. I try to rebulid my website in MVC structure, and i have a problem to call function inside different name sapace(maybe i don't know about OOP very much). Here is my code:

namespace UserFrosting;

class GroupController extends \UserFrosting\BaseController {

    public function testFunction($params){
        //Simple test function that i had error: Call to undefined function UserFrosting\testFunction()
        $params['Password'] = $pw;
        $params['JSON'] = 'Yes';
        $curl = curl_init($url);
        curl_setopt($curl, CURLOPT_POST, true);
        curl_setopt($curl, CURLOPT_POSTFIELDS, http_build_query($params));
        curl_setopt($curl, CURLOPT_TIMEOUT, 30);
        curl_setopt($curl, CURLOPT_RETURNTRANSFER, true); 
        curl_setopt($curl, CURLOPT_VERBOSE, true);
        curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, false);
        $response = curl_exec($curl);
        if (curl_errno($curl)) $obj = (object) array('Result' => 'Error', 'Error' =>      curl_error($curl)); 
        else if (empty($response)) $obj = (object) array('Result' => 'Error', 'Error' => 'Connection failed'); 
        else $obj = json_decode($response);
        curl_close($curl);
        return $obj;
    }  

    public function loginNumber(){
        $params = array("Command" => "SystemStats");
        $api = testFunction($params);
        echo "<h3>Server Status</h3>\r\n";
        if ($api -> Result == "Error") die("Error: " . $api -> Error);
        echo "Logins: " . $api -> Logins . "<br/>\r\n";   
   }
}

So basically i called loginNumber(), then it shows this error:* Call to undefined function UserFrosting\testFunction()*

I had same problem with SoupClient but i shows this error:* Class 'UserFrosting\SoapClient' not found* here is my soapClient code in same namesapce:

 public function templatePost(){

     $client = SoapClient('https://www.test.com/pg/services/WebGate/wsdl', ['encoding' => 'UTF-8']);

     $result = $client->PaymentRequest([
        'MerchantID'     => $MerchantID,
        'Amount'         => $Amount,
        'Description'    => $Description,
        'Email'          => $Email,
        'Mobile'         => $Mobile,
        'CallbackURL'    => $CallbackURL,
     ]);

     if ($result->Status == 100) {
        header('Location: https://www.test.com/pg/StartPay/'.$result->Authority);
     } else {
        echo'ERR: '.$result->Status;
     }
}

I had the same error, what is my problem? how can i call these functions?

Explanation: You do not have a function testFunction, but the object GroupController has a method testFuntion. So you need to use $this->.. since you are already inside that class. Your loginNumber should read:

public function loginNumber(){
    $params = array("Command" => "SystemStats");
    $api = $this->testFunction($params);
    echo "<h3>Server Status</h3>\r\n";
    if ($api -> Result == "Error") die("Error: " . $api -> Error);
    echo "Logins: " . $api -> Logins . "<br/>\r\n";
}

In your second question you are not importing the class SoapClient correctly. Please try:

$client = \SoapClient(...);

This uses SoapClient from root namespace.