diff --git a/src/Assetic/Asset/HttpAsset.php b/src/Assetic/Asset/HttpAsset.php
index bf8b9ba..5a507f9 100644
--- a/src/Assetic/Asset/HttpAsset.php
+++ b/src/Assetic/Asset/HttpAsset.php
@@ -58,11 +58,13 @@ public function load(?FilterInterface $additionalFilter = null)
     public function getLastModified()
     {
         if (false !== @file_get_contents($this->sourceUrl, false, stream_context_create(array('http' => array('method' => 'HEAD'))))) {
-            // TODO: $http_response_header deprecated since PHP 8.5
-            //       Remove when min PHP >= 8.4 and use http_get_last_response_headers() directly
+            // http_get_last_response_headers() was added in PHP 8.4; older versions fall back
+            // to the predefined $http_response_header. The `?? []` coalescing read avoids the
+            // PHP 8.5 deprecation of that variable: a bare read is flagged at compile time,
+            // a coalescing read is not. Drop the fallback once the minimum PHP version is >= 8.4.
             $headers = function_exists('http_get_last_response_headers')
                 ? http_get_last_response_headers()
-                : $http_response_header;
+                : ($http_response_header ?? []);
             foreach ($headers as $header) {
                 if (0 === stripos($header, 'Last-Modified: ')) {
                     list(, $mtime) = explode(':', $header, 2);
diff --git a/src/Assetic/Factory/Loader/BasePhpFormulaLoader.php b/src/Assetic/Factory/Loader/BasePhpFormulaLoader.php
index 75e039f..4c2bf86 100644
--- a/src/Assetic/Factory/Loader/BasePhpFormulaLoader.php
+++ b/src/Assetic/Factory/Loader/BasePhpFormulaLoader.php
@@ -105,7 +105,8 @@ private function processCall($call, array $protoOptions = [])
             $call,
             'echo serialize($_call);',
         )));
-        $args = unserialize(shell_exec('php ' . escapeshellarg($tmp)));
+        $output = shell_exec('php ' . escapeshellarg($tmp));
+        $args = is_string($output) ? unserialize($output) : [];
         unlink($tmp);
 
         $inputs  = isset($args[0]) ? self::argumentToArray($args[0]) : [];
diff --git a/src/Assetic/Filter/CssImportFilter.php b/src/Assetic/Filter/CssImportFilter.php
index 882ab7a..a7abc2f 100644
--- a/src/Assetic/Filter/CssImportFilter.php
+++ b/src/Assetic/Filter/CssImportFilter.php
@@ -7,6 +7,7 @@
 use Assetic\Asset\HttpAsset;
 use Assetic\Factory\AssetFactory;
 use Assetic\Contracts\Filter\DependencyExtractorInterface;
+use Assetic\Contracts\Filter\FilterInterface;
 
 /**
  * Inlines imported stylesheets.