Implement ??= operator

RFC: https://wiki.php.net/rfc/null_coalesce_equal_operator

$a ??= $b is $a ?? ($a = $b), with the difference that $a is only
evaluated once, to the degree that this is possible. In particular
in $a[foo()] ?? $b function foo() is only ever called once.
However, the variable access themselves will be reevaluated.

1 files changed, 14 insertions, 0 deletions

diff --git a/Zend/tests/assign_coalesce_005.phpt b/Zend/tests/assign_coalesce_005.phpt
new file mode 100644
index 0000000000..20510105e1
--- /dev/null
+++ b/Zend/tests/assign_coalesce_005.phpt
@@ -0,0 +1,14 @@
+--TEST--
+Coalesce assign (??=): Cannot reassign $this
+--FILE--
+<?php
+
+class Test {
+    public function foobar() {
+        $this ??= 123;
+    }
+}
+
+?>
+--EXPECTF--
+Fatal error: Cannot re-assign $this in %s on line %d