diff --git a/ext/session/session.c b/ext/session/session.c
index 3d05e9efd3111..8f0acdbcd892b 100644
--- a/ext/session/session.c
+++ b/ext/session/session.c
@@ -2644,7 +2644,14 @@ PHP_FUNCTION(session_start)
 	 * module is unable to rewrite output.
 	 */
 	if (PS(use_cookies) && SG(headers_sent)) {
-		php_error_docref(NULL, E_WARNING, "Session cannot be started after headers have already been sent");
+		/* It's the header sent to blame, not the session in this case */
+		const char *output_start_filename = php_output_get_start_filename();
+		int output_start_lineno = php_output_get_start_lineno();
+		if (output_start_filename != NULL) {
+			php_error_docref(NULL, E_WARNING, "Session cannot be started after headers have already been sent (sent from %s on line %d)", output_start_filename, output_start_lineno);
+		} else {
+			php_error_docref(NULL, E_WARNING, "Session cannot be started after headers have already been sent");
+		}
 		RETURN_FALSE;
 	}
 
diff --git a/ext/session/tests/gh-16372.phpt b/ext/session/tests/gh-16372.phpt
new file mode 100644
index 0000000000000..7ce572792bc3f
--- /dev/null
+++ b/ext/session/tests/gh-16372.phpt
@@ -0,0 +1,19 @@
+--TEST--
+GH-16372: Mention where headers were already sent if session_start fails
+--EXTENSIONS--
+session
+--SKIPIF--
+<?php include('skipif.inc'); ?>
+--FILE--
+<?php
+
+header("X-PHP-Test: test");
+
+echo "Sent headers\n";
+
+session_start();
+?>
+--EXPECTF--
+Sent headers
+
+Warning: session_start(): Session cannot be started after headers have already been sent (sent from %s on line %d) in %s on line %d