diff --git a/composer.json b/composer.json
index 7d135bef..f89420d5 100644
--- a/composer.json
+++ b/composer.json
@@ -15,14 +15,15 @@
         "wp-cli/wp-cli": "^2.12"
     },
     "require-dev": {
-        "wp-cli/wp-cli-tests": "^4"
+        "wp-cli/wp-cli-tests": "^5"
     },
     "config": {
         "process-timeout": 7200,
         "sort-packages": true,
         "allow-plugins": {
             "dealerdirect/phpcodesniffer-composer-installer": true,
-            "johnpbloch/wordpress-core-installer": true
+            "johnpbloch/wordpress-core-installer": true,
+            "phpstan/extension-installer": true
         },
         "lock": false
     },
@@ -51,12 +52,14 @@
         "behat-rerun": "rerun-behat-tests",
         "lint": "run-linter-tests",
         "phpcs": "run-phpcs-tests",
+        "phpstan": "run-phpstan-tests",
         "phpcbf": "run-phpcbf-cleanup",
         "phpunit": "run-php-unit-tests",
         "prepare-tests": "install-package-tests",
         "test": [
             "@lint",
             "@phpcs",
+            "@phpstan",
             "@phpunit",
             "@behat"
         ]
diff --git a/phpstan.neon.dist b/phpstan.neon.dist
new file mode 100644
index 00000000..d2fafd10
--- /dev/null
+++ b/phpstan.neon.dist
@@ -0,0 +1,13 @@
+parameters:
+  level: 9
+  paths:
+    - src
+    - eval-command.php
+  scanDirectories:
+    - vendor/wp-cli/wp-cli/php
+  scanFiles:
+    - vendor/php-stubs/wordpress-stubs/wordpress-stubs.php
+  treatPhpDocTypesAsCertain: false
+  ignoreErrors:
+    - identifier: missingType.parameter
+    - identifier: missingType.return
diff --git a/src/EvalFile_Command.php b/src/EvalFile_Command.php
index 5d2d4b2d..7542f497 100644
--- a/src/EvalFile_Command.php
+++ b/src/EvalFile_Command.php
@@ -72,7 +72,7 @@ private static function execute_eval( $file, $args, $use_include ) {
 		} elseif ( $use_include ) {
 			include $file;
 		} else {
-			$file_contents = file_get_contents( $file );
+			$file_contents = (string) file_get_contents( $file );
 
 			// Adjust for __FILE__ and __DIR__ magic constants.
 			$file_contents = Utils\replace_path_consts( $file_contents, $file );