diff --git a/.travis.yml b/.travis.yml
index 682bc61..a2cd8a4 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,10 +1,18 @@
 language: php
 
 php:
+  - 5.5
+  - 5.6
   - 7.1
   - 7.2
   - nightly
 
+matrix:
+  fast_finish: true
+  allow_failures:
+    - php: 5.5
+    - php: 5.6
+
 install:
   - if [[ "$TRAVIS_PHP_VERSION" != "nightly" ]]; then phpenv config-rm xdebug.ini; fi
   - travis_retry composer self-update
diff --git a/composer.json b/composer.json
index 7536343..00c36a4 100644
--- a/composer.json
+++ b/composer.json
@@ -10,7 +10,7 @@
         }
     ],
     "require": {
-        "php": ">=7.1.0"
+        "php": ">=5.5.9"
     },
     "autoload": {
         "psr-4": {
@@ -18,7 +18,7 @@
         }
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.5",
+        "phpunit/phpunit": "4.8.36|6.5",
         "satooshi/php-coveralls": "~2"
     }
 }
diff --git a/src/Keccak.php b/src/Keccak.php
index c5ad2e8..76b80f5 100644
--- a/src/Keccak.php
+++ b/src/Keccak.php
@@ -3,14 +3,12 @@
 namespace kornrunner;
 
 use Exception;
-use function mb_strlen;
-use function mb_substr;
 
 final class Keccak
 {
-    private const KECCAK_ROUNDS = 24;
-    private const LFSR = 0x01;
-    private const ENCODING = '8bit';
+    const KECCAK_ROUNDS = 24;
+    const LFSR = 0x01;
+    const ENCODING = '8bit';
     private static $keccakf_rotc = [1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 2, 14, 27, 41, 56, 8, 25, 43, 62, 18, 39, 61, 20, 44];
     private static $keccakf_piln = [10, 7, 11, 17, 18, 3, 5, 16, 8, 21, 24, 4, 15, 23, 19, 13, 12,2, 20, 14, 22, 9, 6, 1];
     private static $x64 = (PHP_INT_SIZE === 8);