diff --git a/reference/pthreads/book.xml b/reference/pthreads/book.xml
index 292dbe94a2..ece0cfcfa4 100644
--- a/reference/pthreads/book.xml
+++ b/reference/pthreads/book.xml
@@ -18,6 +18,12 @@
     Threading in PHP should therefore remain to CLI-based applications only.
    </para>
   </warning>
+  <warning>
+   <para>
+    The pthreads extension can only be used with PHP 7.2+. This is due to ZTS
+    mode being unsafe in prior PHP versions.
+   </para>
+  </warning>
   <para>
    The <classname>Threaded</classname> class forms the basis of the
    functionality that allows pthreads to operate. It exposes synchronization