diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index dce925a677..8ca5ca17de 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -4740,7 +4740,9 @@ function reset_mbstring_encoding() {
 }
 
 /**
- * Alternative to filter_var( $var, FILTER_VALIDATE_BOOLEAN ).
+ * Filter/validate a variable as a boolean.
+ *
+ * Alternative to `filter_var( $var, FILTER_VALIDATE_BOOLEAN )`.
  *
  * @since 4.0.0
  *
@@ -4752,7 +4754,7 @@ function wp_validate_boolean( $var ) {
 		return $var;
 	}
 
-	if ( 'false' === $var ) {
+	if ( is_string( $var ) && 'false' === strtolower( $var ) ) {
 		return false;
 	}
 
diff --git a/tests/phpunit/tests/functions/WpValidateBoolean.php b/tests/phpunit/tests/functions/WpValidateBoolean.php
index 812563757e..38a731e31c 100644
--- a/tests/phpunit/tests/functions/WpValidateBoolean.php
+++ b/tests/phpunit/tests/functions/WpValidateBoolean.php
@@ -56,4 +56,20 @@ class Tests_Functions_WpValidateBoolean extends WP_UnitTestCase {
 		// Differs from (bool) conversion.
 		$this->assertFalse( wp_validate_boolean( 'false' ) );
 	}
+
+	/**
+	 * @ticket 30238
+	 */
+	public function test_string_false_uppercase() {
+		// Differs from (bool) conversion.
+		$this->assertFalse( wp_validate_boolean( 'FALSE' ) );
+	}
+
+	/**
+	 * @ticket 30238
+	 */
+	public function test_string_false_mixedcase() {
+		// Differs from (bool) conversion.
+		$this->assertFalse( wp_validate_boolean( 'FaLsE' ) );
+	}
 }