how to unit test curl call in php

How would you go about unit testing a curl implementation?

  public function get() {
    $ch = curl_init($this->request->getUrl());

    curl_setopt($ch, CURLOPT_HEADER, false);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

    $result = curl_exec($ch);
    $code = curl_getinfo($ch, CURLINFO_HTTP_CODE);
    $type = curl_getinfo($ch, CURLINFO_CONTENT_TYPE);
    curl_close($ch);

    if (!strstr($type, 'application/json')) {
      throw new HttpResponseException('JSON response not found');
    }

    return new HttpResponse($code, $result);
  }

I need to test the content type returned so that it can throw an exception.

As thomasrutter suggested, create a class to abstract the usage of the cURL functions.

interface HttpRequest
{
    public function setOption($name, $value);
    public function execute();
    public function getInfo($name);
    public function close();
}

class CurlRequest implements HttpRequest
{
    private $handle = null;

    public function __construct($url) {
        $this->handle = curl_init($url);
    }

    public function setOption($name, $value) {
        curl_setopt($this->handle, $name, $value);
    }

    public function execute() {
        return curl_exec($this->handle);
    }

    public function getInfo($name) {
        return curl_getinfo($this->handle, $name);
    }

    public function close() {
        curl_close($this->handle);
    }
}

Now you can test using a mock of the HttpRequest interface without invoking any of the cURL functions.

public function testGetThrowsWhenContentTypeIsNotJson() {
    $http = $this->getMock('HttpRequest');
    $http->expects($this->any())
         ->method('getInfo')
         ->will($this->returnValue('not JSON'));
    $this->setExpectedException('HttpResponseException');
    // create class under test using $http instead of a real CurlRequest
    $fixture = new ClassUnderTest($http);
    $fixture->get();
}

Edit Fixed simple PHP parse error.

You might use a function mock library. I made one for you: php-mock-phpunit

namespace foo;

use phpmock\phpunit\PHPMock;

class BuiltinTest extends \PHPUnit_Framework_TestCase
{

    use PHPMock;

    public function testCurl()
    {
        $curl_exec = $this->getFunctionMock(__NAMESPACE__, "curl_exec");
        $curl_exec->expects($this->once())->willReturn("body");

        $ch = curl_init();
        $this->assertEquals("body", curl_exec($ch));
    }
}