Use @php() instead of @php [...] @endphp

templates/partials/content-search.blade.php

@@ -1,4 +1,4 @@
-<article @php post_class() @endphp>
+<article @php(post_class())>
   <header>
     <h2 class="entry-title"><a href="{{ get_permalink() }}">{{ get_the_title() }}</a></h2>
     @if (get_post_type() === 'post')
@@ -6,6 +6,6 @@
     @endif
   </header>
   <div class="entry-summary">
-    @php the_excerpt() @endphp
+    @php(the_excerpt())
   </div>
 </article>