Fix strip_tags modifier for falsy input. (#893)

Fixes #890

CHANGELOG.md

@@ -6,6 +6,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- `|strip_tags` does not work if the input is 0 [#890](https://github.com/smarty-php/smarty/issues/890)
+
 ## [4.3.2] - 2023-07-19
 
 ### Fixed

libs/plugins/modifiercompiler.strip_tags.php

@@ -21,7 +21,7 @@
 function smarty_modifiercompiler_strip_tags($params)
 {
     if (!isset($params[ 1 ]) || $params[ 1 ] === true || trim($params[ 1 ], '"') === 'true') {
-        return "preg_replace('!<[^>]*?>!', ' ', {$params[0]} ?: '')";
+        return "preg_replace('!<[^>]*?>!', ' ', (string) {$params[0]})";
     } else {
         return 'strip_tags((string) ' . $params[ 0 ] . ')';
     }

tests/UnitTests/TemplateSource/TagTests/PluginModifier/PluginModifierStripTagsTest.php

@@ -0,0 +1,46 @@
+<?php
+/**
+ * Smarty PHPunit tests of modifier
+ */
+
+namespace UnitTests\TemplateSource\TagTests\PluginModifier;
+use PHPUnit_Smarty;
+
+/**
+ * class for modifier tests
+ *
+ * @runTestsInSeparateProcess
+ * @preserveGlobalState disabled
+ * @backupStaticAttributes enabled
+ */
+class PluginModifierStripTagsTest extends PHPUnit_Smarty {
+
+	public function setUp(): void {
+		$this->setUpSmarty(__DIR__);
+	}
+
+	public function testDefault() {
+		$tpl = $this->smarty->createTemplate('string:{$x|strip_tags}');
+		$tpl->assign('x', '<b>hi</b>');
+		$this->assertEquals(" hi ", $this->smarty->fetch($tpl));
+	}
+
+	public function testParam1() {
+		$tpl = $this->smarty->createTemplate('string:{$x|strip_tags:false}');
+		$tpl->assign('x', '<b>hi</b>');
+		$this->assertEquals("hi", $this->smarty->fetch($tpl));
+	}
+
+	public function testInputIsFalsy0() {
+		$tpl = $this->smarty->createTemplate('string:{$x|strip_tags}');
+		$tpl->assign('x', 0);
+		$this->assertEquals("0", $this->smarty->fetch($tpl));
+	}
+
+	public function testInputIsFalsy1() {
+		$tpl = $this->smarty->createTemplate('string:{$x|strip_tags}');
+		$tpl->assign('x', '');
+		$this->assertEquals("", $this->smarty->fetch($tpl));
+	}
+
+}