Warn if property does not exist and add it to DiContainerTrait (#329)

src/DiContainerTrait.php

@@ -32,6 +32,8 @@
  */
 trait DiContainerTrait
 {
+    use WarnDynamicPropertyTrait;
+
     /**
      * Call from __construct() to initialize the properties allowing
      * developer to pass Dependency Injector Container.

src/Exception.php

@@ -11,6 +11,8 @@
  */
 class Exception extends \Exception
 {
+    use WarnDynamicPropertyTrait;
+
     /** @var array */
     public $params = [];
 

src/WarnDynamicPropertyTrait.php

@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Atk4\Core;
+
+/**
+ * This trait implements https://github.com/php/php-src/pull/7390 for lower PHP versions
+ * and also emit a warning when isset() is called on undefined variable.
+ */
+trait WarnDynamicPropertyTrait
+{
+    protected function warnPropertyDoesNotExist(string $name): void
+    {
+        'trigger_error'('Property ' . static::class . '::$' . $name . ' does not exist', \E_USER_DEPRECATED);
+    }
+
+    public function __isset(string $name): bool
+    {
+        $this->warnPropertyDoesNotExist($name);
+
+        return isset($this->{$name});
+    }
+
+    /**
+     * @return mixed
+     */
+    public function &__get(string $name)
+    {
+        $this->warnPropertyDoesNotExist($name);
+
+        return $this->{$name};
+    }
+
+    /**
+     * @param mixed $value
+     */
+    public function __set(string $name, $value): void
+    {
+        $this->warnPropertyDoesNotExist($name);
+
+        $this->{$name} = $value;
+    }
+
+    public function __unset(string $name): void
+    {
+        $this->warnPropertyDoesNotExist($name);
+
+        unset($this->{$name});
+    }
+}