PHP 8 Support (#782)

Thanks @kornrunner for these changes:
* Test against PHP 8
* U-Tests: FFProbe: OptionsTester: handle phpunit deprecation of at
* FiltersCollection: tweak for call_user_func_array and PHP 8 named arguments

.travis.yml

@@ -30,12 +30,14 @@ matrix:
     - php: 7.2
     - php: 7.3
     - php: 7.4
+    - php: 8.0
+      dist: bionic
     - php: nightly
       env:
         - COMPOSER_DEV_STABILITY=true
 
 before_install:
-  - sudo add-apt-repository ppa:mc3man/trusty-media -y
+  - sudo add-apt-repository ppa:mc3man/${TRAVIS_DIST}-media -y
   - sudo apt-get update -q
   - composer self-update
   - echo "$PHPUNIT_VERSION"

src/FFMpeg/Filters/FiltersCollection.php

@@ -38,7 +38,7 @@ class FiltersCollection implements \Countable, \IteratorAggregate
             return 0;
         }
 
-        return count(call_user_func_array('array_merge', $this->filters));
+        return count(call_user_func_array('array_merge', array_values($this->filters)));
     }
 
     /**
@@ -51,7 +51,7 @@ class FiltersCollection implements \Countable, \IteratorAggregate
                 $this->sorted = $this->filters;
             } else {
                 krsort($this->filters);
-                $this->sorted = call_user_func_array('array_merge', $this->filters);
+                $this->sorted = call_user_func_array('array_merge', array_values($this->filters));
             }
         }
 

tests/Unit/FFProbe/OptionsTesterTest.php

@@ -77,13 +77,11 @@ class OptionsTesterTest extends TestCase
             ->method('fetch')
             ->will($this->returnValue($data));
 
-        $cache->expects($this->at(0))
+        $cache->expects($this->exactly(2))
             ->method('contains')
-            ->will($this->returnValue(false));
-
-        $cache->expects($this->at(1))
-            ->method('contains')
-            ->will($this->returnValue(true));
+            ->willReturnOnConsecutiveCalls(
+                $this->returnValue(false),
+                $this->returnValue(true));
 
         $cache->expects($this->once())
             ->method('save');