diff --git a/.php_cs b/.php_cs
new file mode 100644
index 0000000..f7a56ff
--- /dev/null
+++ b/.php_cs
@@ -0,0 +1,15 @@
+<?php
+$finder = PhpCsFixer\Finder::create()
+    ->exclude('vendor')
+	->exclude('node_modules')
+	->exclude('bin')
+    ->in(__DIR__)
+;
+
+return PhpCsFixer\Config::create()
+    ->setRules([
+        'native_function_invocation' => true,
+		'native_constant_invocation' => true,
+    ])
+    ->setFinder($finder)
+;
diff --git a/.svnignore b/.svnignore
index 8f7beec..93fbfdb 100644
--- a/.svnignore
+++ b/.svnignore
@@ -8,6 +8,7 @@
 .data
 .svnignore
 .wordpress-org
+.php_cs
 Gruntfile.js
 LINGUAS
 Makefile
diff --git a/includes/class-signature.php b/includes/class-signature.php
index ffd5003..3dffb10 100644
--- a/includes/class-signature.php
+++ b/includes/class-signature.php
@@ -53,7 +53,7 @@ class Signature {
 		$config = array(
 			'digest_alg' => 'sha512',
 			'private_key_bits' => 2048,
-			'private_key_type' => OPENSSL_KEYTYPE_RSA,
+			'private_key_type' => \OPENSSL_KEYTYPE_RSA,
 		);
 
 		$key = \openssl_pkey_new( $config );
@@ -91,7 +91,7 @@ class Signature {
 		$signed_string = "(request-target): post $path\nhost: $host\ndate: $date";
 
 		$signature = null;
-		\openssl_sign( $signed_string, $signature, $key, OPENSSL_ALGO_SHA256 );
+		\openssl_sign( $signed_string, $signature, $key, \OPENSSL_ALGO_SHA256 );
 		$signature = \base64_encode( $signature ); // phpcs:ignore
 
 		$key_id = \get_author_posts_url( $user_id ) . '#main-key';
diff --git a/includes/functions.php b/includes/functions.php
index 60d6e3b..982d740 100644
--- a/includes/functions.php
+++ b/includes/functions.php
@@ -89,7 +89,7 @@ function get_webfinger_resource( $user_id ) {
 
 	$user = \get_user_by( 'id', $user_id );
 
-	return $user->user_login . '@' . \wp_parse_url( \home_url(), PHP_URL_HOST );
+	return $user->user_login . '@' . \wp_parse_url( \home_url(), \PHP_URL_HOST );
 }
 
 /**
@@ -253,7 +253,7 @@ function url_to_authorid( $url ) {
 	global $wp_rewrite;
 
 	// check if url hase the same host
-	if ( wp_parse_url( site_url(), PHP_URL_HOST ) !== wp_parse_url( $url, PHP_URL_HOST ) ) {
+	if ( wp_parse_url( site_url(), \PHP_URL_HOST ) !== wp_parse_url( $url, \PHP_URL_HOST ) ) {
 		return 0;
 	}
 
@@ -297,7 +297,7 @@ function url_to_authorid( $url ) {
  */
 function get_blacklist() {
 	$blacklist = \get_option( 'activitypub_blacklist' );
-	$blacklist_hosts  = \explode( PHP_EOL, $blacklist );
+	$blacklist_hosts  = \explode( \PHP_EOL, $blacklist );
 
 	// if no values have been set, revert to the defaults
 	if ( ! $blacklist || ! $blacklist_hosts || ! \is_array( $blacklist_hosts ) ) {
diff --git a/includes/model/class-activity.php b/includes/model/class-activity.php
index 470b192..4aa4107 100644
--- a/includes/model/class-activity.php
+++ b/includes/model/class-activity.php
@@ -69,7 +69,7 @@ class Activity {
 	}
 
 	public function to_json() {
-		return \wp_json_encode( $this->to_array(), JSON_HEX_TAG | JSON_HEX_AMP | JSON_HEX_QUOT );
+		return \wp_json_encode( $this->to_array(), \JSON_HEX_TAG | \JSON_HEX_AMP | \JSON_HEX_QUOT );
 	}
 
 	public function to_simple_array() {
@@ -90,6 +90,6 @@ class Activity {
 	}
 
 	public function to_simple_json() {
-		return \wp_json_encode( $this->to_simple_array(), JSON_HEX_TAG | JSON_HEX_AMP | JSON_HEX_QUOT );
+		return \wp_json_encode( $this->to_simple_array(), \JSON_HEX_TAG | \JSON_HEX_AMP | \JSON_HEX_QUOT );
 	}
 }
diff --git a/includes/model/class-post.php b/includes/model/class-post.php
index a452274..8b9fa65 100644
--- a/includes/model/class-post.php
+++ b/includes/model/class-post.php
@@ -53,7 +53,7 @@ class Post {
 	}
 
 	public function to_json() {
-		return \wp_json_encode( $this->to_array(), JSON_HEX_TAG | JSON_HEX_AMP | JSON_HEX_QUOT );
+		return \wp_json_encode( $this->to_array(), \JSON_HEX_TAG | \JSON_HEX_AMP | \JSON_HEX_QUOT );
 	}
 
 	public function get_attachments() {
@@ -215,7 +215,7 @@ class Post {
 		if ( 'Article' === $this->get_object_type() ) {
 			$title = \get_the_title( $this->post );
 
-			return \html_entity_decode( $title, ENT_QUOTES, 'UTF-8' );
+			return \html_entity_decode( $title, \ENT_QUOTES, 'UTF-8' );
 		}
 
 		return null;
@@ -271,7 +271,7 @@ class Post {
 		$filtered_content = \apply_filters( 'the_content', $content );
 		$filtered_content = \apply_filters( 'activitypub_the_content', $filtered_content, $this->post );
 
-		$decoded_content = \html_entity_decode( $filtered_content, ENT_QUOTES, 'UTF-8' );
+		$decoded_content = \html_entity_decode( $filtered_content, \ENT_QUOTES, 'UTF-8' );
 
 		$allowed_html = \apply_filters( 'activitypub_allowed_html', '<a><p><ul><ol><li><code><blockquote><pre>' );
 
@@ -291,7 +291,7 @@ class Post {
 		$filtered_summary = \apply_filters( 'the_excerpt', $summary );
 		$filtered_summary = \apply_filters( 'activitypub_the_summary', $filtered_summary, $this->post );
 
-		$decoded_summary = \html_entity_decode( $filtered_summary, ENT_QUOTES, 'UTF-8' );
+		$decoded_summary = \html_entity_decode( $filtered_summary, \ENT_QUOTES, 'UTF-8' );
 
 		$allowed_html = \apply_filters( 'activitypub_allowed_html', '<a><p>' );
 
diff --git a/includes/peer/class-followers.php b/includes/peer/class-followers.php
index 9e97a5e..c1846a4 100644
--- a/includes/peer/class-followers.php
+++ b/includes/peer/class-followers.php
@@ -21,7 +21,7 @@ class Followers {
 				isset( $follower['type'] ) &&
 				'Person' === $follower['type'] &&
 				isset( $follower['id'] ) &&
-				false !== \filter_var( $follower['id'], FILTER_VALIDATE_URL )
+				false !== \filter_var( $follower['id'], \FILTER_VALIDATE_URL )
 			) {
 				$followers[ $key ] = $follower['id'];
 			}
@@ -45,7 +45,7 @@ class Followers {
 				isset( $actor['type'] ) &&
 				'Person' === $actor['type'] &&
 				isset( $actor['id'] ) &&
-				false !== \filter_var( $actor['id'], FILTER_VALIDATE_URL )
+				false !== \filter_var( $actor['id'], \FILTER_VALIDATE_URL )
 			) {
 				$actor = $actor['id'];
 			}
diff --git a/includes/rest/class-webfinger.php b/includes/rest/class-webfinger.php
index 049cc2c..2ffe759 100644
--- a/includes/rest/class-webfinger.php
+++ b/includes/rest/class-webfinger.php
@@ -51,7 +51,7 @@ class Webfinger {
 		$resource_identifier = $matches[1];
 		$resource_host = $matches[2];
 
-		if ( \wp_parse_url( \home_url( '/' ), PHP_URL_HOST ) !== $resource_host ) {
+		if ( \wp_parse_url( \home_url( '/' ), \PHP_URL_HOST ) !== $resource_host ) {
 			return new \WP_Error( 'activitypub_wrong_host', \__( 'Resource host does not match blog host', 'activitypub' ), array( 'status' => 404 ) );
 		}
 
diff --git a/templates/json-author.php b/templates/json-author.php
index 59a97e6..bfe4e97 100644
--- a/templates/json-author.php
+++ b/templates/json-author.php
@@ -9,7 +9,7 @@ $json->type = 'Person';
 $json->name = \get_the_author_meta( 'display_name', $author_id );
 $json->summary = \html_entity_decode(
 	\get_the_author_meta( 'description', $author_id ),
-	ENT_QUOTES,
+	\ENT_QUOTES,
 	'UTF-8'
 );
 $json->preferredUsername = \get_the_author_meta( 'login', $author_id ); // phpcs:ignore
@@ -47,8 +47,8 @@ $json->attachment[] = array(
 	'type' => 'PropertyValue',
 	'name' => __( 'Blog', 'activitypub' ),
 	'value' => \html_entity_decode(
-		'<a rel="me" title="' . \esc_attr( \home_url( '/' ) ) . '" target="_blank" href="' . \home_url( '/' ) . '">' . \wp_parse_url( \home_url( '/' ), PHP_URL_HOST ) . '</a>',
-		ENT_QUOTES,
+		'<a rel="me" title="' . \esc_attr( \home_url( '/' ) ) . '" target="_blank" href="' . \home_url( '/' ) . '">' . \wp_parse_url( \home_url( '/' ), \PHP_URL_HOST ) . '</a>',
+		\ENT_QUOTES,
 		'UTF-8'
 	),
 );
@@ -57,8 +57,8 @@ $json->attachment[] = array(
 	'type' => 'PropertyValue',
 	'name' => __( 'Profile', 'activitypub' ),
 	'value' => \html_entity_decode(
-		'<a rel="me" title="' . \esc_attr( \get_author_posts_url( $author_id ) ) . '" target="_blank" href="' . \get_author_posts_url( $author_id ) . '">' . \wp_parse_url( \get_author_posts_url( $author_id ), PHP_URL_HOST ) . '</a>',
-		ENT_QUOTES,
+		'<a rel="me" title="' . \esc_attr( \get_author_posts_url( $author_id ) ) . '" target="_blank" href="' . \get_author_posts_url( $author_id ) . '">' . \wp_parse_url( \get_author_posts_url( $author_id ), \PHP_URL_HOST ) . '</a>',
+		\ENT_QUOTES,
 		'UTF-8'
 	),
 );
@@ -68,8 +68,8 @@ if ( \get_the_author_meta( 'user_url', $author_id ) ) {
 		'type' => 'PropertyValue',
 		'name' => __( 'Website', 'activitypub' ),
 		'value' => \html_entity_decode(
-			'<a rel="me" title="' . \esc_attr( \get_the_author_meta( 'user_url', $author_id ) ) . '" target="_blank" href="' . \get_the_author_meta( 'user_url', $author_id ) . '">' . \wp_parse_url( \get_the_author_meta( 'user_url', $author_id ), PHP_URL_HOST ) . '</a>',
-			ENT_QUOTES,
+			'<a rel="me" title="' . \esc_attr( \get_the_author_meta( 'user_url', $author_id ) ) . '" target="_blank" href="' . \get_the_author_meta( 'user_url', $author_id ) . '">' . \wp_parse_url( \get_the_author_meta( 'user_url', $author_id ), \PHP_URL_HOST ) . '</a>',
+			\ENT_QUOTES,
 			'UTF-8'
 		),
 	);
@@ -92,10 +92,10 @@ $json = \apply_filters( 'activitypub_json_author_array', $json );
 $options = 0;
 // JSON_PRETTY_PRINT added in PHP 5.4
 if ( \get_query_var( 'pretty' ) ) {
-	$options |= JSON_PRETTY_PRINT; // phpcs:ignore
+	$options |= \JSON_PRETTY_PRINT; // phpcs:ignore
 }
 
-$options |= JSON_HEX_TAG | JSON_HEX_AMP | JSON_HEX_QUOT;
+$options |= \JSON_HEX_TAG | \JSON_HEX_AMP | \JSON_HEX_QUOT;
 
 /*
  * Options to be passed to json_encode()
diff --git a/templates/json-post.php b/templates/json-post.php
index ccd9fa9..8761eca 100644
--- a/templates/json-post.php
+++ b/templates/json-post.php
@@ -15,10 +15,10 @@ $json = \apply_filters( 'activitypub_json_post_array', $json );
 $options = 0;
 // JSON_PRETTY_PRINT added in PHP 5.4
 if ( \get_query_var( 'pretty' ) ) {
-	$options |= JSON_PRETTY_PRINT; // phpcs:ignore
+	$options |= \JSON_PRETTY_PRINT; // phpcs:ignore
 }
 
-$options |= JSON_HEX_TAG | JSON_HEX_AMP | JSON_HEX_QUOT;
+$options |= \JSON_HEX_TAG | \JSON_HEX_AMP | \JSON_HEX_QUOT;
 
 /*
  * Options to be passed to json_encode()
diff --git a/templates/settings.php b/templates/settings.php
index ff1d0c7..11c1eb4 100644
--- a/templates/settings.php
+++ b/templates/settings.php
@@ -96,7 +96,7 @@
 
 		<?php
 		// load the existing blacklist from the WordPress options table
-		$activitypub_blacklist = \trim( \implode( PHP_EOL, \ActivityPub\get_blacklist() ), PHP_EOL );
+		$activitypub_blacklist = \trim( \implode( \PHP_EOL, \ActivityPub\get_blacklist() ), \PHP_EOL );
 		?>
 
 		<table class="form-table">
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index e75f0bb..9c550d9 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -1,6 +1,6 @@
 <?php
 
-\error_reporting( E_ALL & ~E_NOTICE & ~E_STRICT & ~E_WARNING );
+\error_reporting( \E_ALL & ~\E_NOTICE & ~\E_STRICT & ~\E_WARNING );
 
 \define( 'WP_DEBUG', false );
 
@@ -18,4 +18,4 @@ tests_add_filter( 'muplugins_loaded', '_manually_load_plugin' );
 
 require $_tests_dir . '/includes/bootstrap.php';
 
-\error_reporting( E_ALL & ~E_NOTICE & ~E_STRICT & ~E_WARNING );
+\error_reporting( \E_ALL & ~\E_NOTICE & ~\E_STRICT & ~\E_WARNING );