diff --git a/resources/functionMap_php84delta.php b/resources/functionMap_php84delta.php
index 4c18488e8b..c5b395c292 100644
--- a/resources/functionMap_php84delta.php
+++ b/resources/functionMap_php84delta.php
@@ -17,6 +17,8 @@
 	'new' => [
 		'http_get_last_response_header ' => ['list<string>'],
 		'http_clear_last_response_header' => ['void'],
+		'mb_lcfirst' => ['string', 'string'=>'string', 'encoding='=>'string'],
+		'mb_ucfirst' => ['string', 'string'=>'string', 'encoding='=>'string'],
 	],
 	'old' => [
 
diff --git a/src/Analyser/TypeSpecifier.php b/src/Analyser/TypeSpecifier.php
index c2d959661e..e65c907163 100644
--- a/src/Analyser/TypeSpecifier.php
+++ b/src/Analyser/TypeSpecifier.php
@@ -1017,9 +1017,9 @@ private function specifyTypesForConstantStringBinaryExpression(
 			&& $exprNode instanceof FuncCall
 			&& $exprNode->name instanceof Name
 			&& in_array(strtolower($exprNode->name->toString()), [
-				'substr', 'strstr', 'stristr', 'strchr', 'strrchr', 'strtolower', 'strtoupper',
-				'mb_substr', 'mb_strstr', 'mb_stristr', 'mb_strchr', 'mb_strrchr', 'mb_strtolower', 'mb_strtoupper',
-				'ucfirst', 'lcfirst', 'ucwords', 'mb_convert_case', 'mb_convert_kana',
+				'substr', 'strstr', 'stristr', 'strchr', 'strrchr', 'strtolower', 'strtoupper', 'ucfirst', 'lcfirst',
+				'mb_substr', 'mb_strstr', 'mb_stristr', 'mb_strchr', 'mb_strrchr', 'mb_strtolower', 'mb_strtoupper', 'mb_ucfirst', 'mb_lcfirst',
+				'ucwords', 'mb_convert_case', 'mb_convert_kana',
 			], true)
 			&& isset($exprNode->getArgs()[0])
 			&& $constantType->getValue() !== ''
diff --git a/src/Type/Php/StrCaseFunctionsReturnTypeExtension.php b/src/Type/Php/StrCaseFunctionsReturnTypeExtension.php
index df1dfc88e9..ebb693672d 100644
--- a/src/Type/Php/StrCaseFunctionsReturnTypeExtension.php
+++ b/src/Type/Php/StrCaseFunctionsReturnTypeExtension.php
@@ -34,6 +34,8 @@ class StrCaseFunctionsReturnTypeExtension implements DynamicFunctionReturnTypeEx
 		'mb_strtolower' => 1,
 		'lcfirst' => 1,
 		'ucfirst' => 1,
+		'mb_lcfirst' => 1,
+		'mb_ucfirst' => 1,
 		'ucwords' => 1,
 		'mb_convert_case' => 2,
 		'mb_convert_kana' => 1,
diff --git a/tests/PHPStan/Analyser/NodeScopeResolverTest.php b/tests/PHPStan/Analyser/NodeScopeResolverTest.php
index 7146d8e31d..a134571f9e 100644
--- a/tests/PHPStan/Analyser/NodeScopeResolverTest.php
+++ b/tests/PHPStan/Analyser/NodeScopeResolverTest.php
@@ -1082,6 +1082,9 @@ public function dataFileAsserts(): iterable
 		if (PHP_VERSION_ID >= 70300) {
 			yield from $this->gatherAssertTypes(__DIR__ . '/data/str-casing.php');
 		}
+		if (PHP_VERSION_ID >= 80400) {
+			yield from $this->gatherAssertTypes(__DIR__ . '/data/str-caseing-php84.php');
+		}
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/non-empty-string-substr-specifying.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/unset-conditional-expressions.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/conditional-types-inference.php');
diff --git a/tests/PHPStan/Analyser/data/str-caseing-php84.php b/tests/PHPStan/Analyser/data/str-caseing-php84.php
new file mode 100644
index 0000000000..ed6e8dab73
--- /dev/null
+++ b/tests/PHPStan/Analyser/data/str-caseing-php84.php
@@ -0,0 +1,23 @@
+<?php
+
+namespace StrCasingReturnTypePhp84;
+
+use function PHPStan\Testing\assertType;
+
+class Foo {
+	/**
+	 * @param numeric-string $numericS
+	 * @param non-empty-string $nonE
+	 * @param literal-string $literal
+	 */
+	public function bar($numericS, $nonE, $literal) {
+		assertType("'aBC'", mb_lcfirst('ABC'));
+		assertType("'Abc'", mb_ucfirst('abc'));
+		assertType("numeric-string", mb_lcfirst($numericS));
+		assertType("numeric-string", mb_ucfirst($numericS));
+		assertType("non-empty-string", mb_lcfirst($nonE));
+		assertType("non-empty-string", mb_ucfirst($nonE));
+		assertType("string", mb_lcfirst($literal));
+		assertType("string", mb_ucfirst($literal));
+	}
+}