diff --git a/phpstan.neon.dist b/phpstan.neon.dist
new file mode 100644
index 00000000..229d3401
--- /dev/null
+++ b/phpstan.neon.dist
@@ -0,0 +1,15 @@
+parameters:
+  level: 9
+  paths:
+    - src
+    - shell-command.php
+  scanDirectories:
+    - vendor/wp-cli/wp-cli/php
+  scanFiles:
+    - vendor/php-stubs/wordpress-stubs/wordpress-stubs.php
+  treatPhpDocTypesAsCertain: false
+  ignoreErrors:
+    - identifier: class.notFound
+    - identifier: missingType.property
+    - identifier: missingType.parameter
+    - identifier: missingType.return
diff --git a/src/Shell_Command.php b/src/Shell_Command.php
index 66c03fa4..edd2c611 100644
--- a/src/Shell_Command.php
+++ b/src/Shell_Command.php
@@ -27,15 +27,15 @@ class Shell_Command extends WP_CLI_Command {
 	 *     => string(6) "WP-CLI"
 	 */
 	public function __invoke( $_, $assoc_args ) {
+		$class = WP_CLI\Shell\REPL::class;
+
 		$implementations = array(
-			'Psy\\Shell',
-			'Boris\\Boris',
-			'WP_CLI\\Shell\\REPL',
+			\Psy\Shell::class,
+			\Boris\Boris::class,
+			WP_CLI\Shell\REPL::class,
 		);
 
-		if ( Utils\get_flag_value( $assoc_args, 'basic' ) ) {
-			$class = 'WP_CLI\\Shell\\REPL';
-		} else {
+		if ( ! Utils\get_flag_value( $assoc_args, 'basic' ) ) {
 			foreach ( $implementations as $candidate ) {
 				if ( class_exists( $candidate ) ) {
 					$class = $candidate;
@@ -44,10 +44,17 @@ public function __invoke( $_, $assoc_args ) {
 			}
 		}
 
-		if ( 'Psy\\Shell' === $class ) {
+		/**
+		 * @var class-string $class
+		 */
+
+		if ( \Psy\Shell::class === $class ) {
 			$shell = new Psy\Shell();
 			$shell->run();
 		} else {
+			/**
+			 * @var class-string<WP_CLI\Shell\REPL> $class
+			 */
 			$repl = new $class( 'wp> ' );
 			$repl->start();
 		}
diff --git a/src/WP_CLI/Shell/REPL.php b/src/WP_CLI/Shell/REPL.php
index cc9ee064..15697bf0 100644
--- a/src/WP_CLI/Shell/REPL.php
+++ b/src/WP_CLI/Shell/REPL.php
@@ -17,6 +17,7 @@ public function __construct( $prompt ) {
 	}
 
 	public function start() {
+		// @phpstan-ignore while.alwaysTrue
 		while ( true ) {
 			$line = $this->prompt();
 
@@ -30,7 +31,7 @@ public function start() {
 				ob_start();
 				// phpcs:ignore Squiz.PHP.Eval.Discouraged -- This is meant to be a REPL, no way to avoid eval.
 				eval( $line );
-				$out = ob_get_clean();
+				$out = (string) ob_get_clean();
 				if ( 0 < strlen( $out ) ) {
 					$out = rtrim( $out, "\n" ) . "\n";
 				}
@@ -44,13 +45,13 @@ public function start() {
 				ob_start();
 				// phpcs:ignore Squiz.PHP.Eval.Discouraged -- This is meant to be a REPL, no way to avoid eval.
 				$evl = eval( $line );
-				$out = ob_get_clean();
+				$out = (string) ob_get_clean();
 				if ( 0 < strlen( $out ) ) {
 					echo rtrim( $out, "\n" ) . "\n";
 				}
 				echo '=> ';
 				var_dump( $evl );
-				fwrite( STDOUT, ob_get_clean() );
+				fwrite( STDOUT, (string) ob_get_clean() );
 			}
 		}
 	}
@@ -82,13 +83,16 @@ private function prompt() {
 
 		$done = false;
 		do {
+			// @phpstan-ignore booleanNot.alwaysTrue
 			$prompt = ( ! $done && false !== $full_line ) ? '--> ' : $this->prompt;
 
 			$fp = popen( self::create_prompt_cmd( $prompt, $this->history_file ), 'r' );
 
-			$line = fgets( $fp );
+			$line = $fp ? fgets( $fp ) : '';
 
-			pclose( $fp );
+			if ( $fp ) {
+				pclose( $fp );
+			}
 
 			if ( ! $line ) {
 				break;