gen_stub: strip dollar sign escape sequence

C doesn't have this, and will throw the following warning/error:

```
error: unknown escape sequence '\$' [-Werror,-Wunknown-escape-sequence]
```

Since adding this to zend_test for CI's sake, this will bomb out. Strip
this specific sequence out to make cc happy.

Author: NattyNarwhal

build/gen_stub.php

@@ -2348,16 +2348,23 @@ public function getCExpr(): ?string
             // The string in $expr has had the octal, hex, and unicode
             // backslash sequences already applied for double-quoted strings,
             // but not the other sequences.
+            //
             // PHP has single quote strings, C doesn't (they're one char).
             // Single-quoted strings need handling to replace their escapes
             // with the double-quoted equivalent; namely single quote escapes.
+            //
             // Double-quoted strings have similar escape sequences as C does,
-            // so we can pass them through directly.
+            // so we can pass them through directly. However, C does *not*
+            // support the \$ escape sequence (in ""), so strip that. Variable
+            // interpolation shouldn't be possible in a stub, so we don't need
+            // to worry about mangling such a case.
             if (preg_match("/(^'|'$)/", $expr)) {
                 $expr = substr($expr, 1, -1); // strip quotes, readd later
                 $expr = str_replace("\\'", "'", $expr);
                 $expr = addcslashes($expr, "\\\"");
                 $expr = "\"$expr\"";
+            } else {
+                $expr = str_replace('\$', "$", $expr);
             }
         }
         return $expr[0] == '"' ? $expr : preg_replace('(\bnull\b)', 'NULL', str_replace('\\', '', $expr));

ext/zend_test/test.stub.php

@@ -63,6 +63,7 @@ class _ZendTestClass implements _ZendTestInterface {
 	 * the generated header won't compile. */
         public static string $doubleQuoteEscaped = "BEGIN \n\r\t\v\e\f\\\$\"\101\x41\u{41} END";
         public static string $singleQuoteEscaped = 'BEGIN \n\r\t\v\e\f\\\$\"\101\x41\u{41} END';
+        public static string $escapeInterpolated = "begin \$ \\$ end";
 
         public int $intProp = 123;
         public ?stdClass $classProp = null;