diff --git a/src/Rules/ClassCaseSensitivityCheck.php b/src/Rules/ClassCaseSensitivityCheck.php
index 27cfdb52dbc..240ee04d8a3 100644
--- a/src/Rules/ClassCaseSensitivityCheck.php
+++ b/src/Rules/ClassCaseSensitivityCheck.php
@@ -8,9 +8,10 @@
 use PHPStan\Parser\UseAliasVisitor;
 use PHPStan\Reflection\ReflectionProvider;
 use function count;
-use function implode;
 use function sprintf;
+use function strrpos;
 use function strtolower;
+use function substr;
 
 #[AutowiredService]
 final class ClassCaseSensitivityCheck
@@ -105,15 +106,18 @@ public static function getNameAsWritten(Name $node): string
 			return $resolvedName;
 		}
 
-		$originalParts = $originalName->getParts();
-		if (count($originalParts) !== 1) {
+		if (count($originalName->getParts()) !== 1) {
 			return $resolvedName;
 		}
 
-		$resolvedParts = $node->getParts();
-		$resolvedParts[count($resolvedParts) - 1] = $originalParts[0];
+		// the original name takes over the last part of the resolved name,
+		// everything up to the last separator stays as resolved
+		$lastSeparatorPos = strrpos($resolvedName, '\\');
+		if ($lastSeparatorPos === false) {
+			return $originalName->toString();
+		}
 
-		return implode('\\', $resolvedParts);
+		return substr($resolvedName, 0, $lastSeparatorPos + 1) . $originalName->toString();
 	}
 
 }
diff --git a/src/Rules/FunctionDefinitionCheck.php b/src/Rules/FunctionDefinitionCheck.php
index c1666ebdacd..1e7f68cb5c4 100644
--- a/src/Rules/FunctionDefinitionCheck.php
+++ b/src/Rules/FunctionDefinitionCheck.php
@@ -863,18 +863,17 @@ private function getOriginalClassNamePairsFromTypeNode(Identifier|Name|ComplexTy
 				return [];
 			}
 
-			$originalParts = $originalName->getParts();
+			$originalPartsCount = count($originalName->getParts());
 			$resolvedParts = $typeNode->getParts();
-
-			$originalPartsCount = count($originalParts);
 			$resolvedPartsCount = count($resolvedParts);
 
-			if ($originalPartsCount <= $resolvedPartsCount) {
-				$prefixParts = array_slice($resolvedParts, 0, $resolvedPartsCount - $originalPartsCount);
-				$originalCaseClassName = implode('\\', array_merge($prefixParts, $originalParts));
-			} else {
-				$originalCaseClassName = $originalName->toString();
-			}
+			// the original name takes over the last parts of the resolved name,
+			// the parts it does not cover stay as resolved
+			$prefixParts = $originalPartsCount <= $resolvedPartsCount
+				? array_slice($resolvedParts, 0, $resolvedPartsCount - $originalPartsCount)
+				: [];
+			$prefix = $prefixParts === [] ? '' : implode('\\', $prefixParts) . '\\';
+			$originalCaseClassName = $prefix . $originalName->toString();
 
 			if ($originalCaseClassName === $resolvedName) {
 				return [];
diff --git a/src/Type/Php/ImplodeFunctionReturnTypeExtension.php b/src/Type/Php/ImplodeFunctionReturnTypeExtension.php
index 34f4080a9f5..da376bc3429 100644
--- a/src/Type/Php/ImplodeFunctionReturnTypeExtension.php
+++ b/src/Type/Php/ImplodeFunctionReturnTypeExtension.php
@@ -15,6 +15,7 @@
 use PHPStan\Type\Constant\ConstantArrayType;
 use PHPStan\Type\Constant\ConstantStringType;
 use PHPStan\Type\DynamicFunctionReturnTypeExtension;
+use PHPStan\Type\IntegerRangeType;
 use PHPStan\Type\IntersectionType;
 use PHPStan\Type\StringType;
 use PHPStan\Type\Type;
@@ -85,9 +86,14 @@ private function implode(Type $arrayType, Type $separatorType): Type
 		$accessoryTypes = [];
 		$valueTypeAsString = $arrayType->getIterableValueType()->toString();
 		if ($arrayType->isIterableAtLeastOnce()->yes()) {
-			if ($valueTypeAsString->isNonFalsyString()->yes() || $separatorType->isNonFalsyString()->yes()) {
+			// The separator only appears between elements, so it can only
+			// guarantee a non-empty/non-falsy result when the array has at
+			// least two elements. A single-element array drops the separator
+			// entirely (e.g. implode(',', ['']) === '').
+			$separatorAppears = IntegerRangeType::createAllGreaterThanOrEqualTo(2)->isSuperTypeOf($arrayType->getArraySize())->yes();
+			if ($valueTypeAsString->isNonFalsyString()->yes() || ($separatorAppears && $separatorType->isNonFalsyString()->yes())) {
 				$accessoryTypes[] = new AccessoryNonFalsyStringType();
-			} elseif ($valueTypeAsString->isNonEmptyString()->yes() || $separatorType->isNonEmptyString()->yes()) {
+			} elseif ($valueTypeAsString->isNonEmptyString()->yes() || ($separatorAppears && $separatorType->isNonEmptyString()->yes())) {
 				$accessoryTypes[] = new AccessoryNonEmptyStringType();
 			}
 		}
diff --git a/tests/PHPStan/Analyser/nsrt/implode.php b/tests/PHPStan/Analyser/nsrt/implode.php
index ecd36a0570f..da52efbe56d 100644
--- a/tests/PHPStan/Analyser/nsrt/implode.php
+++ b/tests/PHPStan/Analyser/nsrt/implode.php
@@ -78,6 +78,49 @@ public function unsealedConstArr($unsealed) {
 		assertType('non-falsy-string', implode(',', $unsealed));
 	}
 
+	/**
+	 * @param non-empty-array<string> $arr
+	 * @param non-empty-list<string> $list
+	 */
+	public function nonEmptyArrayOfStrings(array $arr, array $list) {
+		// A non-empty array can have a single element, which drops the
+		// separator entirely (implode(',', ['']) === ''), so the non-falsy
+		// separator must not make the whole result non-falsy.
+		assertType('string', implode(',', $arr));
+		assertType('string', implode('', $arr));
+		assertType('string', implode(',', $list));
+	}
+
+	/**
+	 * @param non-empty-array<non-empty-string> $arr
+	 */
+	public function nonEmptyArrayOfNonEmptyStrings(array $arr) {
+		assertType('non-empty-string', implode(',', $arr));
+		assertType('non-empty-string', implode('', $arr));
+	}
+
+	/**
+	 * @param array{string, string} $arr
+	 * @param array{string, string, string} $arr3
+	 */
+	public function atLeastTwoElements(array $arr, array $arr3) {
+		// At least two elements guarantees the separator appears.
+		assertType('non-falsy-string', implode(',', $arr));
+		assertType('non-falsy-string', implode(',', $arr3));
+		assertType('string', implode('', $arr));
+	}
+
+	public function bug5521() {
+		// https://github.com/phpstan/phpstan/issues/5521
+		// A single-element array drops the separator entirely, so the result
+		// is just the (empty) single value, not a non-empty-string.
+		$foo = [null];
+		assertType("''", implode(',', $foo));
+
+		$bar = [''];
+		assertType("''", implode(',', $bar));
+	}
+
 	/** @param array{'a', 'b', ...<int, string>} $unsealed */
 	public function unsealedConstArrEmptySeparator($unsealed) {
 		// Empty separator + a possibly-empty unsealed value type leaves no
diff --git a/tests/PHPStan/Analyser/nsrt/non-empty-string.php b/tests/PHPStan/Analyser/nsrt/non-empty-string.php
index c8031310aee..02e9c7f22ad 100644
--- a/tests/PHPStan/Analyser/nsrt/non-empty-string.php
+++ b/tests/PHPStan/Analyser/nsrt/non-empty-string.php
@@ -182,7 +182,7 @@ public function doFoo2(string $s, array $nonEmptyArrayWithStrings): void
 	{
 		assertType('string', implode($s, $nonEmptyArrayWithStrings));
 		assertType('string', implode('', $nonEmptyArrayWithStrings));
-		assertType('non-falsy-string', implode(' ', $nonEmptyArrayWithStrings));
+		assertType('string', implode(' ', $nonEmptyArrayWithStrings));
 		assertType('string', implode($nonEmptyArrayWithStrings));
 	}
 
@@ -204,7 +204,7 @@ public function doFoo4(string $s, array $nonEmptyArrayWithNonEmptyStrings): void
 	{
 		assertType('non-empty-string', implode($s, $nonEmptyArrayWithNonEmptyStrings));
 		assertType('non-empty-string', implode('', $nonEmptyArrayWithNonEmptyStrings));
-		assertType('non-falsy-string', implode(' ', $nonEmptyArrayWithNonEmptyStrings));
+		assertType('non-empty-string', implode(' ', $nonEmptyArrayWithNonEmptyStrings));
 		assertType('non-empty-string', implode($nonEmptyArrayWithNonEmptyStrings));
 	}
 
@@ -227,7 +227,9 @@ public function nonE($glue, array $a)
 			return "xyz";
 		}
 
-		assertType('non-empty-string', implode($glue, $a));
+		// A single-element array drops the glue entirely, so the result can
+		// still be an empty/falsy string even with a non-empty glue.
+		assertType('string', implode($glue, $a));
 	}
 
 	public function sayHello2(int $i): void
@@ -248,7 +250,7 @@ public function nonE2($glue, array $a)
 			return "xyz";
 		}
 
-		assertType('non-empty-string', join($glue, $a));
+		assertType('string', join($glue, $a));
 	}
 
 }
diff --git a/tests/PHPStan/Analyser/nsrt/non-falsy-string.php b/tests/PHPStan/Analyser/nsrt/non-falsy-string.php
index 12ca4c3fac4..1e4d87d89e5 100644
--- a/tests/PHPStan/Analyser/nsrt/non-falsy-string.php
+++ b/tests/PHPStan/Analyser/nsrt/non-falsy-string.php
@@ -78,7 +78,7 @@ function concat(string $s, string $nonFalsey, $numericS, $nonEmpty, $literalStri
 	function stringFunctions(string $s, $nonFalsey, $arrayOfNonFalsey, $nonEmptyArray, array $arr)
 	{
 		assertType('string', implode($nonFalsey, []));
-		assertType('non-falsy-string', implode($nonFalsey, $nonEmptyArray));
+		assertType('string', implode($nonFalsey, $nonEmptyArray));
 		assertType('non-falsy-string', implode($nonFalsey, $arrayOfNonFalsey));
 		assertType('non-falsy-string', implode($s, $arrayOfNonFalsey));