how to unit test curl call in php

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.