diff --git a/src/Type/Php/StrlenFunctionTypeSpecifyingExtension.php b/src/Type/Php/StrlenFunctionTypeSpecifyingExtension.php
new file mode 100644
index 0000000000..ff074b92c2
--- /dev/null
+++ b/src/Type/Php/StrlenFunctionTypeSpecifyingExtension.php
@@ -0,0 +1,65 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Type\Php;
+
+use PhpParser\Node\Expr\FuncCall;
+use PHPStan\Analyser\Scope;
+use PHPStan\Analyser\SpecifiedTypes;
+use PHPStan\Analyser\TypeSpecifier;
+use PHPStan\Analyser\TypeSpecifierAwareExtension;
+use PHPStan\Analyser\TypeSpecifierContext;
+use PHPStan\DependencyInjection\AutowiredService;
+use PHPStan\Reflection\FunctionReflection;
+use PHPStan\Type\Accessory\AccessoryNonEmptyStringType;
+use PHPStan\Type\FunctionTypeSpecifyingExtension;
+use PHPStan\Type\StaticTypeFactory;
+use function count;
+use function in_array;
+
+#[AutowiredService]
+final class StrlenFunctionTypeSpecifyingExtension implements FunctionTypeSpecifyingExtension, TypeSpecifierAwareExtension
+{
+
+	private TypeSpecifier $typeSpecifier;
+
+	public function isFunctionSupported(
+		FunctionReflection $functionReflection,
+		FuncCall $node,
+		TypeSpecifierContext $context,
+	): bool
+	{
+		return !$context->null()
+			&& count($node->getArgs()) >= 1
+			&& in_array($functionReflection->getName(), ['strlen', 'mb_strlen'], true);
+	}
+
+	public function specifyTypes(
+		FunctionReflection $functionReflection,
+		FuncCall $node,
+		Scope $scope,
+		TypeSpecifierContext $context,
+	): SpecifiedTypes
+	{
+		if (!$scope->getType($node->getArgs()[0]->value)->isString()->yes()) {
+			return new SpecifiedTypes([], []);
+		}
+
+		$argSpecifiedTypes = $this->typeSpecifier->create($node->getArgs()[0]->value, new AccessoryNonEmptyStringType(), $context, $scope);
+
+		if ($context->truthy()) {
+			return $this->typeSpecifier->create($node, StaticTypeFactory::falsey(), TypeSpecifierContext::createFalse(), $scope)
+				->setRootExpr($node)
+				->unionWith($argSpecifiedTypes);
+		}
+
+		return $this->typeSpecifier->create($node, StaticTypeFactory::truthy(), TypeSpecifierContext::createFalse(), $scope)
+			->setRootExpr($node)
+			->unionWith($argSpecifiedTypes);
+	}
+
+	public function setTypeSpecifier(TypeSpecifier $typeSpecifier): void
+	{
+		$this->typeSpecifier = $typeSpecifier;
+	}
+
+}
diff --git a/tests/PHPStan/Analyser/nsrt/bug-13688.php b/tests/PHPStan/Analyser/nsrt/bug-13688.php
new file mode 100644
index 0000000000..91a6c2af57
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-13688.php
@@ -0,0 +1,140 @@
+<?php declare(strict_types = 1);
+
+namespace Bug13688Nsrt;
+
+use function PHPStan\Testing\assertType;
+
+function doFoo(string $input): void
+{
+	$inputLen = strlen($input);
+	assertType('string', $input);
+
+	if ($inputLen > 0) {
+		assertType('non-empty-string', $input);
+	}
+}
+
+function doBar(string $s): void
+{
+	$len = strlen($s);
+	if ($len) {
+		assertType('non-empty-string', $s);
+	} else {
+		assertType("''", $s);
+	}
+}
+
+/**
+ * @param ''|':' $input
+ */
+function doBaz(string $input): void
+{
+	$inputLen = strlen($input);
+	if ($inputLen > 0) {
+		assertType("':'", $input);
+		assertType('1', $inputLen);
+	}
+}
+
+function directTruthy(string $s): void
+{
+	if (strlen($s)) {
+		assertType('non-empty-string', $s);
+	} else {
+		assertType("''", $s);
+	}
+	assertType('string', $s);
+}
+
+function directNegation(string $s): void
+{
+	if (!strlen($s)) {
+		assertType("''", $s);
+	} else {
+		assertType('non-empty-string', $s);
+	}
+	assertType('string', $s);
+}
+
+function mbStrlenTruthy(string $s): void
+{
+	if (mb_strlen($s)) {
+		assertType('non-empty-string', $s);
+	} else {
+		assertType("''", $s);
+	}
+}
+
+function mbStrlenNegation(string $s): void
+{
+	if (!mb_strlen($s)) {
+		assertType("''", $s);
+	} else {
+		assertType('non-empty-string', $s);
+	}
+}
+
+function booleanAnd(string $a, string $b): void
+{
+	if (strlen($a) && strlen($b)) {
+		assertType('non-empty-string', $a);
+		assertType('non-empty-string', $b);
+	}
+}
+
+function booleanOr(string $a, string $b): void
+{
+	if (!strlen($a) || !strlen($b)) {
+		return;
+	}
+	assertType('non-empty-string', $a);
+	assertType('non-empty-string', $b);
+}
+
+function ternary(string $s): void
+{
+	$result = strlen($s) ? 'non-empty' : 'empty';
+	assertType("'empty'|'non-empty'", $result);
+}
+
+/** @param int|string $intOrString */
+function nonStringInput($intOrString): void
+{
+	if (strlen($intOrString)) {
+		assertType('int|string', $intOrString);
+	}
+}
+
+/** @param float|string $floatOrString */
+function nonStringInputFloat($floatOrString): void
+{
+	if (strlen($floatOrString)) {
+		assertType('float|string', $floatOrString);
+	}
+}
+
+/** @param bool|string $boolOrString */
+function nonStringInputBool($boolOrString): void
+{
+	if (strlen($boolOrString)) {
+		assertType('bool|string', $boolOrString);
+	}
+}
+
+function looseComparisonTrue(string $s): void
+{
+	if (strlen($s) == true) {
+		assertType('non-empty-string', $s);
+	} else {
+		assertType("''", $s);
+	}
+}
+
+function looseComparisonFalse(string $s): void
+{
+	if (strlen($s) == false) {
+		assertType("''", $s);
+	} else {
+		assertType('non-empty-string', $s);
+	}
+}
diff --git a/tests/PHPStan/Analyser/nsrt/narrow-cast.php b/tests/PHPStan/Analyser/nsrt/narrow-cast.php
index 5687c0b23a..b86b3781b2 100644
--- a/tests/PHPStan/Analyser/nsrt/narrow-cast.php
+++ b/tests/PHPStan/Analyser/nsrt/narrow-cast.php
@@ -7,9 +7,9 @@
 /** @param array<mixed> $arr */
 function doFoo(string $x, array $arr): void {
 	if ((bool) strlen($x)) {
-		assertType('string', $x); // could be non-empty-string
+		assertType('non-empty-string', $x);
 	} else {
-		assertType('string', $x);
+		assertType("''", $x);
 	}
 	assertType('string', $x);
 
diff --git a/tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php b/tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php
index 31570cfb12..4eb98a13a1 100644
--- a/tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php
+++ b/tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php
@@ -1343,4 +1343,9 @@ public function testArrayFindKeyExisting(): void
 		]);
 	}
 
+	public function testBug13688(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-13688.php'], []);
+	}
+
 }
diff --git a/tests/PHPStan/Rules/Arrays/data/bug-13688.php b/tests/PHPStan/Rules/Arrays/data/bug-13688.php
new file mode 100644
index 0000000000..19a7d57a7a
--- /dev/null
+++ b/tests/PHPStan/Rules/Arrays/data/bug-13688.php
@@ -0,0 +1,12 @@
+<?php declare(strict_types = 1);
+
+namespace Bug13688;
+
+$inputs = [ '', ':' ];
+
+foreach ( $inputs as $input )
+{
+	$inputLen = \strlen($input);
+	$hasTrailingColon = $inputLen > 0 && $input[$inputLen-1] === ':';
+	echo $hasTrailingColon ? "{$input} has trailing colon\n" : "{$input} does not have trailing colon\n";
+}