Rename $_ to $decl when assigned

We're making $_ shorthand for 'unused', so the typechecker has started
to complain about this.

Author: fredemmott

src/core/ComposableElement.php

@@ -520,17 +520,17 @@ final protected function __transferContext(
    */
   protected static function &__xhpAttributeDeclaration(
   ): array<string, array<int, mixed>> {
-    static $_ = array();
-    return $_;
+    static $decl = array();
+    return $decl;
   }
 
   /**
    * Defined in elements by the `category` keyword. This is just a list of all
    * categories an element belongs to. Each category is a key with value 1.
    */
   protected function &__xhpCategoryDeclaration(): array<string, int> {
-    static $_ = array();
-    return $_;
+    static $decl = array();
+    return $decl;
   }
 
   /**
@@ -541,8 +541,8 @@ protected function &__xhpCategoryDeclaration(): array<string, int> {
    * biggest mess you've ever seen.
    */
   protected function &__xhpChildrenDeclaration(): mixed {
-    static $_ = 1;
-    return $_;
+    static $decl = 1;
+    return $decl;
   }
 
   /**