diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e82642aa5..21977687c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -28,6 +28,7 @@ jobs:
           - "7.3"
           - "7.4"
           - "8.0"
+          - "8.1"
 
         compiler:
           - default
diff --git a/libs/sysplugins/smarty_internal_cacheresource_file.php b/libs/sysplugins/smarty_internal_cacheresource_file.php
index abed98d85..c77ae9e17 100644
--- a/libs/sysplugins/smarty_internal_cacheresource_file.php
+++ b/libs/sysplugins/smarty_internal_cacheresource_file.php
@@ -196,8 +196,8 @@ public function clear(Smarty $smarty, $resource_name, $cache_id, $compile_id, $e
      */
     public function hasLock(Smarty $smarty, Smarty_Template_Cached $cached)
     {
-        clearstatcache(true, $cached->lock_id);
-        if (is_file($cached->lock_id)) {
+        clearstatcache(true, $cached->lock_id ?? '');
+        if (null !== $cached->lock_id && is_file($cached->lock_id)) {
             $t = filemtime($cached->lock_id);
             return $t && (time() - $t < $smarty->locking_timeout);
         } else {
diff --git a/libs/sysplugins/smarty_internal_compile_function.php b/libs/sysplugins/smarty_internal_compile_function.php
index d0f2b0f4a..84e9584d9 100644
--- a/libs/sysplugins/smarty_internal_compile_function.php
+++ b/libs/sysplugins/smarty_internal_compile_function.php
@@ -157,7 +157,7 @@ public function compile($args, Smarty_Internal_TemplateCompilerBase $compiler)
             $output = "<?php echo \"/*%%SmartyNocache:{$compiler->template->compiled->nocache_hash}%%*/<?php ";
             $output .= "\\\$_smarty_tpl->smarty->ext->_tplFunction->restoreTemplateVariables(\\\$_smarty_tpl, '{$_name}');?>\n";
             $output .= "/*/%%SmartyNocache:{$compiler->template->compiled->nocache_hash}%%*/\";\n?>";
-            $output .= "<?php echo str_replace('{$compiler->template->compiled->nocache_hash}', \$_smarty_tpl->compiled->nocache_hash, ob_get_clean());\n";
+            $output .= "<?php echo str_replace('{$compiler->template->compiled->nocache_hash}', \$_smarty_tpl->compiled->nocache_hash ?? '', ob_get_clean());\n";
             $output .= "}\n}\n";
             $output .= "/*/ {$_funcName}_nocache */\n\n";
             $output .= "?>\n";
diff --git a/libs/sysplugins/smarty_internal_config_file_compiler.php b/libs/sysplugins/smarty_internal_config_file_compiler.php
index 90c5dcefa..a9b940e5a 100644
--- a/libs/sysplugins/smarty_internal_config_file_compiler.php
+++ b/libs/sysplugins/smarty_internal_config_file_compiler.php
@@ -158,7 +158,7 @@ public function compileTemplate(Smarty_Internal_Template $template)
         }
         // template header code
         $template_header =
-            "<?php /* Smarty version " . Smarty::SMARTY_VERSION . ", created on " . strftime("%Y-%m-%d %H:%M:%S") .
+            "<?php /* Smarty version " . Smarty::SMARTY_VERSION . ", created on " . date("Y-m-d H:i:s") .
             "\n";
         $template_header .= "         compiled from '{$this->template->source->filepath}' */ ?>\n";
         $code = '<?php $_smarty_tpl->smarty->ext->configLoad->_loadConfigVars($_smarty_tpl, ' .
diff --git a/libs/sysplugins/smarty_internal_runtime_codeframe.php b/libs/sysplugins/smarty_internal_runtime_codeframe.php
index 983ca6180..b5361c9bb 100644
--- a/libs/sysplugins/smarty_internal_runtime_codeframe.php
+++ b/libs/sysplugins/smarty_internal_runtime_codeframe.php
@@ -45,7 +45,7 @@ public function create(
             $properties[ 'cache_lifetime' ] = $_template->cache_lifetime;
         }
         $output = "<?php\n";
-        $output .= "/* Smarty version {$properties[ 'version' ]}, created on " . strftime("%Y-%m-%d %H:%M:%S") .
+        $output .= "/* Smarty version {$properties[ 'version' ]}, created on " . date("Y-m-d H:i:s") .
                    "\n  from '" . str_replace('*/', '* /', $_template->source->filepath) . "' */\n\n";
         $output .= "/* @var Smarty_Internal_Template \$_smarty_tpl */\n";
         $dec = "\$_smarty_tpl->_decodeProperties(\$_smarty_tpl, " . var_export($properties, true) . ',' .
diff --git a/libs/sysplugins/smarty_internal_templatecompilerbase.php b/libs/sysplugins/smarty_internal_templatecompilerbase.php
index 6a8dc9c48..272616148 100644
--- a/libs/sysplugins/smarty_internal_templatecompilerbase.php
+++ b/libs/sysplugins/smarty_internal_templatecompilerbase.php
@@ -1135,7 +1135,7 @@ public function trigger_template_error($args = null, $line = null, $tagline = nu
             flush();
         }
         $e = new SmartyCompilerException($error_text);
-        $e->line = $line;
+        $e->setLine($line);
         $e->source = trim(preg_replace('![\t\r\n]+!', ' ', $match[ $line - 1 ]));
         $e->desc = $args;
         $e->template = $this->template->source->filepath;
diff --git a/libs/sysplugins/smartycompilerexception.php b/libs/sysplugins/smartycompilerexception.php
index f7ad39b93..8833aa52c 100644
--- a/libs/sysplugins/smartycompilerexception.php
+++ b/libs/sysplugins/smartycompilerexception.php
@@ -16,12 +16,12 @@ public function __toString()
     }
 
     /**
-     * The line number of the template error
-     *
-     * @type int|null
+     * @param int $line
      */
-    public $line = null;
-
+    public function setLine($line)
+    {
+        $this->line = $line;
+    }
     /**
      * The template source snippet relating to the error
      *
diff --git a/tests/UnitTests/CacheResourceTests/_shared/CacheResourceTestCommon.php b/tests/UnitTests/CacheResourceTests/_shared/CacheResourceTestCommon.php
index 92207d2ad..dc4c1cd7e 100644
--- a/tests/UnitTests/CacheResourceTests/_shared/CacheResourceTestCommon.php
+++ b/tests/UnitTests/CacheResourceTests/_shared/CacheResourceTestCommon.php
@@ -30,7 +30,8 @@ protected function doClearCacheAssertion($a, $b)
 
     public function compiledPrefilter($text, Smarty_Internal_Template $tpl)
     {
-        return str_replace('#', $tpl->getTemplateVars('test'), $text);
+        $replace = $tpl->getTemplateVars('test');
+        return str_replace('#', $replace ?? '', $text);
     }
 
     /**
diff --git a/tests/UnitTests/ResourceTests/Stream/StreamResourceTest.php b/tests/UnitTests/ResourceTests/Stream/StreamResourceTest.php
index 6d0201bd2..402f0226e 100644
--- a/tests/UnitTests/ResourceTests/Stream/StreamResourceTest.php
+++ b/tests/UnitTests/ResourceTests/Stream/StreamResourceTest.php
@@ -234,7 +234,7 @@ public function stream_write($data)
         $v = &$GLOBALS[$this->varname];
         $l = strlen($data);
         $p = &$this->position;
-        $v = substr($v, 0, $p) . $data . substr($v, $p += $l);
+        $v = substr($v ?? '', 0, $p) . $data . substr($v ?? '', $p += $l);
 
         return $l;
     }
diff --git a/tests/UnitTests/SecurityTests/SecurityTest.php b/tests/UnitTests/SecurityTests/SecurityTest.php
index 4d1b8a3ab..d68ac3d48 100644
--- a/tests/UnitTests/SecurityTests/SecurityTest.php
+++ b/tests/UnitTests/SecurityTests/SecurityTest.php
@@ -366,9 +366,9 @@ public function testNotTrustedUri()
 
     /**
      * In security mode, accessing $smarty.template_object should be illegal.
-     * @expectedException SmartyCompilerException
      */
     public function testSmartyTemplateObject() {
+        $this->expectException(SmartyCompilerException::class);
         $this->smarty->display('string:{$smarty.template_object}');
     }
 
@@ -416,7 +416,7 @@ public function stream_write($data)
         $v = &$GLOBALS[$this->varname];
         $l = strlen($data);
         $p = &$this->position;
-        $v = substr($v, 0, $p) . $data . substr($v, $p += $l);
+        $v = substr($v ?? '', 0, $p) . $data . substr($v ?? '', $p += $l);
 
         return $l;
     }
diff --git a/tests/UnitTests/TemplateSource/TagTests/TemplateFunction/CompileFunctionTest.php b/tests/UnitTests/TemplateSource/TagTests/TemplateFunction/CompileFunctionTest.php
index ca3e37141..61a085838 100644
--- a/tests/UnitTests/TemplateSource/TagTests/TemplateFunction/CompileFunctionTest.php
+++ b/tests/UnitTests/TemplateSource/TagTests/TemplateFunction/CompileFunctionTest.php
@@ -435,9 +435,9 @@ public function dataTestSpacing()
 
     /**
      * Test handling of function names that are a security risk
-     * @expectedException        SmartyCompilerException
      */
     public function testIllegalFunctionName() {
+        $this->expectException(SmartyCompilerException::class);
 	    $this->smarty->fetch('string:{function name=\'rce(){};echo "hi";function \'}{/function}');
     }
 
diff --git a/tests/UnitTests/TemplateSource/ValueTests/Variables/Stream/StreamVariableTest.php b/tests/UnitTests/TemplateSource/ValueTests/Variables/Stream/StreamVariableTest.php
index e49d62058..18560ba82 100644
--- a/tests/UnitTests/TemplateSource/ValueTests/Variables/Stream/StreamVariableTest.php
+++ b/tests/UnitTests/TemplateSource/ValueTests/Variables/Stream/StreamVariableTest.php
@@ -96,7 +96,7 @@ public function stream_write($data)
         $v = &$GLOBALS[$this->varname];
         $l = strlen($data);
         $p = &$this->position;
-        $v = substr($v, 0, $p) . $data . substr($v, $p += $l);
+        $v = substr($v ?? '', 0, $p) . $data . substr($v ?? '', $p += $l);
 
         return $l;
     }