travis: Rewrite checks and add PHP compatibility tests

- Remove HHVM

HHVM recently announced ending PHP support:
https://hhvm.com/blog/2018/09/12/end-of-php-support-future-of-hack.html

"HHVM v3.30 will be the last release series where HHVM aims to support
PHP. [...] Ultimately, we recommend that projects either migrate
entirely to the Hack language, or entirely to PHP7 and the PHP runtime."

RSS-Bridge never "officially" supported HHVM, so support can be removed.

- Use composer for all versions

PHP 5.6 is using PEAR, while all other versions use Composer to manage
packages and dependencies. This commit removes PEAR for PHP 5.6 in favor
of Composer. This also simplifies the script.

- Add PHP compatibility tests

Uses https://github.com/PHPCompatibility/PHPCompatibility

RSS-Bridge supports PHP 5.6 or higher. This commit adds tests to check
compatibility and detect breaking changes.

"phpcompatibility.xml" contains the ruleset.

Notice: Technically RSS-Bridge requires PHP 5.6.1, but for some reason
PHPCompatibility doesn't accept "5.6.1" for "testVersion". This is why
INI_SCANNER_TYPED is excluded from tests.

- Rearrange tests

PHP 5.6:
  - Coding style (phpcs.xml)
  - PHP compatibility (phpcompatibility.xml)

PHP 7.0:
  - Coding style (phpcs.xml)
  - Unit tests (phpunit.xml) - using stable release of PHPUnit

PHP nightly:
  - Coding style (phpcs.xml)
  - Unit tests (phpunit.xml) - using latest version of PHPUnit
  - PHP compatibility - no exceptions for PHP 5.6+

- Documentation added to improve maintainability

2 files changed, 66 insertions, 12 deletions

diff --git a/.travis.yml b/.travis.yml
index 5c746a4d..80f141b9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,26 +3,35 @@ sudo: false
 language: php
 
 install:
-  - if [[ $TRAVIS_PHP_VERSION == "hhvm" ]]; then
-      composer global require squizlabs/PHP_CodeSniffer;
-    else
-      pear channel-update pear.php.net;
-      pear install PHP_CodeSniffer;
-    fi
+  - composer global require dealerdirect/phpcodesniffer-composer-installer;
+  - composer global require phpcompatibility/php-compatibility;
+  # Use PHPUnit 6 for unit tests (stable), requires PHP 7
   - if [[ $TRAVIS_PHP_VERSION == "7.0" ]]; then
       composer global require phpunit/phpunit ^6;
     fi
+  # Use latest PHPUnit on nightly to detect breaking changes
+  - if [[ $TRAVIS_PHP_VERSION == "nightly" ]]; then
+      composer global require phpunit/phpunit;
+    fi
 
 script:
   - phpenv rehash
-  - if [[ $TRAVIS_PHP_VERSION == "hhvm" ]]; then
-      /home/travis/.composer/vendor/bin/phpcs . --standard=phpcs.xml --warning-severity=0 --extensions=php -p;
-    else
-      phpcs . --standard=phpcs.xml --warning-severity=0 --extensions=php -p;
+  # Run PHP_CodeSniffer on all versions
+  - ~/.composer/vendor/bin/phpcs . --standard=phpcs.xml --warning-severity=0 --extensions=php -p;
+  # Check PHP compatibility for the lowest supported version
+  - if [[ $TRAVIS_PHP_VERSION == "5.6" ]]; then
+      ~/.composer/vendor/bin/phpcs . --standard=phpcompatibility.xml --warning-severity=0 --extensions=php -p;
     fi
+  # Run unit tests (stable)
   - if [[ $TRAVIS_PHP_VERSION == "7.0" ]]; then
       phpunit --configuration=phpunit.xml --include-path=lib/;
     fi
+  # Run unit tests (latest/nightly)
+  # Check PHP compatibility for all versions, starting at the lowest supported version in order to detect breaking changes
+  - if [[ $TRAVIS_PHP_VERSION == "nightly" ]]; then
+      phpunit --configuration=phpunit.xml --include-path=lib/;
+      ~/.composer/vendor/bin/phpcs . --standard=PHPCompatibility --warning-severity=0 --extensions=php -p --runtime-set testVersion 5.6-;
+    fi
 
 matrix:
   fast_finish: true
@@ -30,9 +39,7 @@ matrix:
   include:
     - php: 5.6
     - php: 7.0
-    - php: hhvm
     - php: nightly
 
   allow_failures:
-    - php: hhvm
     - php: nightly
diff --git a/phpcompatibility.xml b/phpcompatibility.xml
new file mode 100644
index 00000000..f232523f
--- /dev/null
+++ b/phpcompatibility.xml
@@ -0,0 +1,47 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ruleset name="RSS-Bridge PHPCompatibility">
+  <description>Defines rules for PHPCompatibility</description>
+  <exclude-pattern>./static</exclude-pattern>
+  <exclude-pattern>./vendor</exclude-pattern>
+
+  <!-- Run against the PHPCompatibility ruleset -->
+  <!--
+
+  -->
+  <config name="testVersion" value="5.6"/>
+  <rule ref="PHPCompatibility">
+    <!--
+    "PHP 7 changes how most errors are reported by PHP. Instead of reporting
+    errors through the traditional error reporting mechanism used by PHP 5,
+    most errors are now reported by throwing Error exceptions."
+
+    from: http://php.net/manual/en/language.errors.php7.php
+
+    Skip this check for PHP 5.6 in order to support PHP 7.x
+
+    Catch Exception and Error separately to support both versions.
+
+    Example:
+
+    <code>
+		try {
+			// Run your code here
+		} catch(Error $e) {
+			// Handle errors (PHP 7.0+)
+		} catch(Exception $e) {
+			// Handle exceptions (PHP 5.6+)
+		}
+    </code>
+    -->
+    <exclude name="PHPCompatibility.Classes.NewClasses.errorFound"/>
+    <!--
+    RSS-Bridge uses parse_ini_file with INI_SCANNER_TYPED to load configuration
+    settings. INI_SCANNER_TYPED was added in PHP 5.6.1. Skip checking for that
+    specific constant.
+
+    References: http://php.net/manual/de/function.parse-ini-file.php
+    -->
+    <exclude name="PHPCompatibility.Constants.NewConstants.ini_scanner_typedFound"/>
+  </rule>
+
+</ruleset>