diff --git a/.travis.yml b/.travis.yml
index 71b9734..e6f8615 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,10 +3,9 @@ sudo: false
 language: php
 
 php:
-  - 7.0
-  - 7.1
-  - 7.2
-  - 7.3
+#  - 7.2
+#  - 7.3
+  - 7.4
   - nightly
 
 matrix:
diff --git a/lib/Internal/HttpsSocket.php b/lib/Internal/HttpsSocket.php
index 4af5cac..40d99b3 100644
--- a/lib/Internal/HttpsSocket.php
+++ b/lib/Internal/HttpsSocket.php
@@ -70,7 +70,7 @@ final class HttpsSocket extends Socket
                 $request->setBody($data);
                 $request->setHeader('content-type', 'application/dns-message');
                 $request->setHeader('accept', 'application/dns-message');
-                $request->setHeader('content-length', strlen($data));
+                $request->setHeader('content-length', \strlen($data));
                 $request->setHeaders($this->nameserver->getHeaders());
                 break;
             case Nameserver::GOOGLE_JSON:
diff --git a/lib/Internal/Socket.php b/lib/Internal/Socket.php
index 5debc5a..8428dde 100644
--- a/lib/Internal/Socket.php
+++ b/lib/Internal/Socket.php
@@ -146,7 +146,7 @@ abstract class Socket
             $message = "Unexpected error during resolution: ".$exception->getMessage();
             $exception = new DnsException($message, 0, $exception);
         }
-        
+
         $pending = $this->pending;
         $this->pending = [];
 
diff --git a/lib/Rfc8484StubResolver.php b/lib/Rfc8484StubResolver.php
index 8376d4f..65ccf2c 100644
--- a/lib/Rfc8484StubResolver.php
+++ b/lib/Rfc8484StubResolver.php
@@ -153,7 +153,7 @@ final class Rfc8484StubResolver implements Resolver
                                     foreach ($reason->getReasons() as $reason) {
                                         $reasons []= $reason->getMessage();
                                     }
-                                    $error .= " (".implode(", ", $reasons).")";
+                                    $error .= " (".\implode(", ", $reasons).")";
                                 }
                                 $errors[] = $error;
                             }
@@ -263,7 +263,7 @@ final class Rfc8484StubResolver implements Resolver
             $attemptDescription = [];
 
             $exceptions = [];
-            
+
             while ($attempt < $attempts) {
                 try {
                     $attemptDescription[] = $nameserver;