diff --git a/src/Type/Php/Base64DecodeDynamicFunctionReturnTypeExtension.php b/src/Type/Php/Base64DecodeDynamicFunctionReturnTypeExtension.php
index bf91435e62..34f1856c6f 100644
--- a/src/Type/Php/Base64DecodeDynamicFunctionReturnTypeExtension.php
+++ b/src/Type/Php/Base64DecodeDynamicFunctionReturnTypeExtension.php
@@ -6,13 +6,15 @@
 use PHPStan\Analyser\Scope;
 use PHPStan\DependencyInjection\AutowiredService;
 use PHPStan\Reflection\FunctionReflection;
+use PHPStan\TrinaryLogic;
 use PHPStan\Type\BenevolentUnionType;
 use PHPStan\Type\Constant\ConstantBooleanType;
+use PHPStan\Type\Constant\ConstantStringType;
 use PHPStan\Type\DynamicFunctionReturnTypeExtension;
-use PHPStan\Type\MixedType;
 use PHPStan\Type\StringType;
 use PHPStan\Type\Type;
 use PHPStan\Type\UnionType;
+use function base64_decode;
 
 #[AutowiredService]
 final class Base64DecodeDynamicFunctionReturnTypeExtension implements DynamicFunctionReturnTypeExtension
@@ -29,32 +31,41 @@ public function getTypeFromFunctionCall(
 		Scope $scope,
 	): Type
 	{
-		if (!isset($functionCall->getArgs()[1])) {
+		if (!isset($functionCall->getArgs()[0])) {
 			return new StringType();
 		}
 
-		$argType = $scope->getType($functionCall->getArgs()[1]->value);
-
-		if ($argType instanceof MixedType) {
-			return new BenevolentUnionType([new StringType(), new ConstantBooleanType(false)]);
+		$stringArgNode = $functionCall->getArgs()[0]->value;
+		$constantStrings = $scope->getType($stringArgNode)->getConstantStrings();
+		if ($constantStrings !== []) {
+			$isValidBase64 = TrinaryLogic::lazyExtremeIdentity(
+				$constantStrings,
+				static function (ConstantStringType $constantString): TrinaryLogic {
+					$isValid = base64_decode($constantString->getValue(), true) !== false;
+					return TrinaryLogic::createFromBoolean($isValid);
+				},
+			);
+		} else {
+			$isValidBase64 = TrinaryLogic::createMaybe();
 		}
 
-		$isTrueType = $argType->isTrue();
-		$isFalseType = $argType->isFalse();
-		$compareTypes = $isTrueType->compareTo($isFalseType);
-		if ($compareTypes === $isTrueType) {
-			return new UnionType([new StringType(), new ConstantBooleanType(false)]);
+		if (isset($functionCall->getArgs()[1])) {
+			$strictArgNode = $functionCall->getArgs()[1]->value;
+			$isStrict = $scope->getType($strictArgNode)->toBoolean()->toTrinaryLogic();
+		} else {
+			$isStrict = TrinaryLogic::createNo();
 		}
-		if ($compareTypes === $isFalseType) {
+
+		if ($isStrict->no() || $isValidBase64->yes()) {
 			return new StringType();
 		}
-
-		// second argument could be interpreted as true
-		if (!$isTrueType->no()) {
-			return new UnionType([new StringType(), new ConstantBooleanType(false)]);
+		if ($isStrict->yes() && $isValidBase64->no()) {
+			return new ConstantBooleanType(false);
 		}
-
-		return new StringType();
+		if ($isStrict->maybe() && $isValidBase64->maybe()) {
+			return new BenevolentUnionType([new StringType(), new ConstantBooleanType(false)]);
+		}
+		return new UnionType([new StringType(), new ConstantBooleanType(false)]);
 	}
 
 }
diff --git a/tests/PHPStan/Analyser/data/functions.php b/tests/PHPStan/Analyser/data/functions.php
index 01b6d33f3d..07b83f827f 100644
--- a/tests/PHPStan/Analyser/data/functions.php
+++ b/tests/PHPStan/Analyser/data/functions.php
@@ -78,11 +78,12 @@
 $fileObject = new \SplFileObject(__FILE__);
 $fileObjectStat = $fileObject->fstat();
 
-$base64DecodeWithoutStrict = base64_decode('');
-$base64DecodeWithStrictDisabled = base64_decode('', false);
-$base64DecodeWithStrictEnabled = base64_decode('', true);
-$base64DecodeDefault = base64_decode('', null);
-$base64DecodeBenevolent = base64_decode('', $undefined);
+$string = (string)time();
+$base64DecodeWithoutStrict = base64_decode($string);
+$base64DecodeWithStrictDisabled = base64_decode($string, false);
+$base64DecodeWithStrictEnabled = base64_decode($string, true);
+$base64DecodeDefault = base64_decode($string, null);
+$base64DecodeBenevolent = base64_decode($string, $undefined);
 
 
 //str_word_count
diff --git a/tests/PHPStan/Analyser/nsrt/base64_decode.php b/tests/PHPStan/Analyser/nsrt/base64_decode.php
index 34de145d9a..d19d03524b 100644
--- a/tests/PHPStan/Analyser/nsrt/base64_decode.php
+++ b/tests/PHPStan/Analyser/nsrt/base64_decode.php
@@ -11,11 +11,27 @@ public function nonStrictMode(string $string): void
 	{
 		assertType('string', base64_decode($string));
 		assertType('string', base64_decode($string, false));
+		assertType('string', base64_decode($string, 0));
+		assertType('string', base64_decode('UEhQU3Rhbg==', false));
+		assertType('string', base64_decode('!!!', false));
 	}
 
 	public function strictMode(string $string): void
 	{
 		assertType('string|false', base64_decode($string, true));
+		assertType('string|false', base64_decode($string, 1));
+		assertType('string', base64_decode(mt_rand() ? 'UEhQU3Rhbg==' : 'cm9ja3Mh', true));
+		assertType('string|false', base64_decode(mt_rand() ? 'UEhQU3Rhbg==' : '!!!', true));
+		assertType('false', base64_decode(mt_rand() ? '!' : '!!!', true));
+	}
+
+	public function mixedMode(string $string): void
+	{
+		assertType('(string|false)', base64_decode($string, unknown()));
+		assertType('(string|false)', base64_decode($string, mt_rand(0, 1) === 1));
+		assertType('(string|false)', base64_decode($string, mt_rand(0, 1)));
+		assertType('string', base64_decode('UEhQU3Rhbg==', mt_rand(0, 1) === 1));
+		assertType('string|false', base64_decode('!!!', mt_rand(0, 1) === 1));
 	}
 
 }