diff --git a/src/Type/Php/Base64DecodeDynamicFunctionReturnTypeExtension.php b/src/Type/Php/Base64DecodeDynamicFunctionReturnTypeExtension.php
index f12d949d86..1167f5e3fc 100644
--- a/src/Type/Php/Base64DecodeDynamicFunctionReturnTypeExtension.php
+++ b/src/Type/Php/Base64DecodeDynamicFunctionReturnTypeExtension.php
@@ -2,17 +2,22 @@
 
 namespace PHPStan\Type\Php;
 
+use PhpParser\Node\Expr;
 use PhpParser\Node\Expr\FuncCall;
 use PHPStan\Analyser\Scope;
 use PHPStan\DependencyInjection\AutowiredService;
 use PHPStan\Reflection\FunctionReflection;
 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\TypeCombinator;
 use PHPStan\Type\UnionType;
+use function base64_decode;
+use function count;
 
 #[AutowiredService]
 final class Base64DecodeDynamicFunctionReturnTypeExtension implements DynamicFunctionReturnTypeExtension
@@ -30,10 +35,14 @@ public function getTypeFromFunctionCall(
 	): Type
 	{
 		$args = $functionCall->getArgs();
-		if (!isset($args[1])) {
+		if (!isset($args[0])) {
 			return new StringType();
 		}
 
+		if (!isset($args[1])) {
+			return $this->resolveType($scope, $args[0]->value, false) ?? new StringType();
+		}
+
 		$argType = $scope->getType($args[1]->value);
 
 		if ($argType instanceof MixedType) {
@@ -44,10 +53,11 @@ public function getTypeFromFunctionCall(
 		$isFalseType = $argType->isFalse();
 		$compareTypes = $isTrueType->compareTo($isFalseType);
 		if ($compareTypes === $isTrueType) {
-			return new UnionType([new StringType(), new ConstantBooleanType(false)]);
+			return $this->resolveType($scope, $args[0]->value, true)
+				?? new UnionType([new StringType(), new ConstantBooleanType(false)]);
 		}
 		if ($compareTypes === $isFalseType) {
-			return new StringType();
+			return $this->resolveType($scope, $args[0]->value, false) ?? new StringType();
 		}
 
 		// second argument could be interpreted as true
@@ -55,7 +65,34 @@ public function getTypeFromFunctionCall(
 			return new UnionType([new StringType(), new ConstantBooleanType(false)]);
 		}
 
-		return new StringType();
+		return $this->resolveType($scope, $args[0]->value, false) ?? new StringType();
+	}
+
+	private function resolveType(Scope $scope, Expr $stringArg, bool $strict): ?Type
+	{
+		$constantStrings = $scope->getType($stringArg)->getConstantStrings();
+		if (count($constantStrings) === 0) {
+			return null;
+		}
+
+		$resultTypes = [];
+		foreach ($constantStrings as $constantString) {
+			$decoded = base64_decode($constantString->getValue(), true);
+			if ($decoded === false) {
+				// In non-strict mode base64_decode is lenient about invalid input,
+				// so leave the result as a generic string instead of guessing the value.
+				if (!$strict) {
+					return null;
+				}
+
+				$resultTypes[] = new ConstantBooleanType(false);
+				continue;
+			}
+
+			$resultTypes[] = new ConstantStringType($decoded);
+		}
+
+		return TypeCombinator::union(...$resultTypes);
 	}
 
 }
diff --git a/tests/PHPStan/Analyser/nsrt/base64_decode.php b/tests/PHPStan/Analyser/nsrt/base64_decode.php
index 34de145d9a..5d912457b2 100644
--- a/tests/PHPStan/Analyser/nsrt/base64_decode.php
+++ b/tests/PHPStan/Analyser/nsrt/base64_decode.php
@@ -18,4 +18,13 @@ public function strictMode(string $string): void
 		assertType('string|false', base64_decode($string, true));
 	}
 
+	public function constantInput(): void
+	{
+		assertType("'Hello world'", base64_decode('SGVsbG8gd29ybGQ='));
+		assertType("'Hello world'", base64_decode('SGVsbG8gd29ybGQ=', false));
+		assertType("'Hello world'", base64_decode('SGVsbG8gd29ybGQ=', true));
+		assertType('false', base64_decode('not valid base64 @@@', true));
+		assertType('string', base64_decode('not valid base64 @@@', false));
+	}
+
 }
diff --git a/tests/PHPStan/Analyser/nsrt/functions.php b/tests/PHPStan/Analyser/nsrt/functions.php
index af90a6a67d..ba6a7cdfb1 100644
--- a/tests/PHPStan/Analyser/nsrt/functions.php
+++ b/tests/PHPStan/Analyser/nsrt/functions.php
@@ -165,10 +165,10 @@
 assertType('array{0: int, 1: int, 2: int, 3: int, 4: int, 5: int, 6: int, 7: int, 8: int, 9: int, 10: int, 11: int, 12: int, dev: int, ino: int, mode: int, nlink: int, uid: int, gid: int, rdev: int, size: int, atime: int, mtime: int, ctime: int, blksize: int, blocks: int}|false', $lstat);
 assertType('array{0: int, 1: int, 2: int, 3: int, 4: int, 5: int, 6: int, 7: int, 8: int, 9: int, 10: int, 11: int, 12: int, dev: int, ino: int, mode: int, nlink: int, uid: int, gid: int, rdev: int, size: int, atime: int, mtime: int, ctime: int, blksize: int, blocks: int}|false', $fstat);
 assertType('array{0: int, 1: int, 2: int, 3: int, 4: int, 5: int, 6: int, 7: int, 8: int, 9: int, 10: int, 11: int, 12: int, dev: int, ino: int, mode: int, nlink: int, uid: int, gid: int, rdev: int, size: int, atime: int, mtime: int, ctime: int, blksize: int, blocks: int}', $fileObjectStat);
-assertType('string', $base64DecodeWithoutStrict);
-assertType('string', $base64DecodeWithStrictDisabled);
-assertType('string|false', $base64DecodeWithStrictEnabled);
-assertType('string', $base64DecodeDefault);
+assertType("''", $base64DecodeWithoutStrict);
+assertType("''", $base64DecodeWithStrictDisabled);
+assertType("''", $base64DecodeWithStrictEnabled);
+assertType("''", $base64DecodeDefault);
 assertType('(string|false)', $base64DecodeBenevolent);
 assertType('*ERROR*', $strWordCountWithoutParameters);
 assertType('*ERROR*', $strWordCountWithTooManyParams);