Add regression tests for bug #79836

Co-authored-by: changochen1@gmail.com

Author: nielsdos

Zend/tests/bug79836.phpt

@@ -0,0 +1,16 @@
+--TEST--
+Bug #79836 (Segfault in concat_function)
+--INI--
+error_reporting = E_ALL & ~E_WARNING
+--FILE--
+<?php
+ob_start(function () use (&$c) {
+	$c = 0;
+}, 1);
+$c .= [];
+$c .= [];
+ob_end_clean();
+echo "Done\n";
+?>
+--EXPECT--
+Done

Zend/tests/bug79836_1.phpt

@@ -0,0 +1,18 @@
+--TEST--
+Bug #79836 (Segfault in concat_function)
+--INI--
+error_reporting = E_ALL & ~E_WARNING
+--FILE--
+<?php
+$x = 'non-empty';
+ob_start(function () use (&$c) {
+	$c = 0;
+}, 1);
+$c = [];
+$x = $c . $x;
+$x = $c . $x;
+ob_end_clean();
+echo "Done\n";
+?>
+--EXPECT--
+Done

Zend/tests/bug79836_2.phpt

@@ -0,0 +1,27 @@
+--TEST--
+Bug #79836 (Segfault in concat_function)
+--INI--
+error_reporting = E_ALL & ~E_WARNING
+--FILE--
+<?php
+$c = str_repeat("abcd", 10);
+
+ob_start(function () use (&$c) {
+	$c = 0;
+}, 1);
+
+class X {
+	function __toString() {
+		echo "a";
+		return "abc";
+	}
+}
+
+$xxx = new X;
+
+$x = $c . $xxx;
+ob_end_clean();
+echo $x . "\n";
+?>
+--EXPECT--
+abcdabcdabcdabcdabcdabcdabcdabcdabcdabcdabc