[php] Set required PHP version to 7.2 (#6603)

* Set PHP 7.2 as minimum version

* Update PHPUnit to 8 || 9

* Set Bionic environment in Travis config

* PHPUnit 8 requires void return in static methods

* PHPUnit 8 requires void return in static methods

* Fix curl exception message test

When I run "curl http://wrong_host.zxc" output is:
curl: (6) Could not resolve host: wrong_host.zxc
Maybe this message is different across versions.

Tested curl version:
curl 7.54.0 (x86_64-apple-darwin18.0) libcurl/7.54.0 LibreSSL/2.6.5 zlib/1.2.11 nghttp2/1.24.1

* Update assertions of deprecated assertInternalType

* Migrate to expectException method of PHPUnit 8

* Fix PHPCS Fixer errors

* Replace deprecated 'assertRegExp' assertion

* Exclude PHPUnit and php-cs-fixer cache

* Refresh samples

* Set root Travis CI environment to PHP 7.2.5

* Change to 7.3 as 7.2.27 is highest preinstalled

* Fix testWrongHost test

samples/client/petstore/php/OpenAPIClient-php/tests/OrderApiTest.php

@@ -8,7 +8,7 @@ class OrderApiTest extends TestCase
 {
 
     // add a new pet (id 10005) to ensure the pet object is available for all the tests
-    public static function setUpBeforeClass()
+    public static function setUpBeforeClass(): void
     {
         // for error reporting (need to run with php5.3 to get no warning)
         //ini_set('display_errors', 1);
@@ -31,13 +31,11 @@ class OrderApiTest extends TestCase
         $order->setStatus("placed");
         $this->assertSame("placed", $order->getStatus());
     }
- 
-    /**
-     * @expectedException InvalidArgumentException
-     */
+
     public function testOrderException()
     {
         // initialize the API client
+        $this->expectException(\InvalidArgumentException::class);
         $order = new Model\Order();
         $order->setStatus("invalid_value");
     }