diff --git a/php/src/Invoice.php b/php/src/Invoice.php
index 6b5d82c..596945f 100644
--- a/php/src/Invoice.php
+++ b/php/src/Invoice.php
@@ -1,16 +1,24 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Theatrical;
 
 class Invoice
 {
-    public string $customer;
-    public array $performances;
+    /**
+     * @var string
+     */
+    public $customer;
 
-    public function __construct($customer, $performances)
+    /**
+     * @var array
+     */
+    public $performances;
+
+    public function __construct(string $customer, array $performances)
     {
         $this->customer = $customer;
         $this->performances = $performances;
     }
-    
-}
\ No newline at end of file
+}
diff --git a/php/src/Performance.php b/php/src/Performance.php
index c0fb631..fb97997 100644
--- a/php/src/Performance.php
+++ b/php/src/Performance.php
@@ -1,15 +1,29 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Theatrical;
 
-class Performance 
+class Performance
 {
-    public string $play_id;
-    public int $audience;
+    /**
+     * @var string
+     */
+    public $play_id;
 
-    public function __construct($play_id, $audience)
+    /**
+     * @var int
+     */
+    public $audience;
+
+    /**
+     * @var Play
+     */
+    public $play;
+
+    public function __construct(string $play_id, int $audience)
     {
         $this->play_id = $play_id;
         $this->audience = $audience;
     }
-}
\ No newline at end of file
+}
diff --git a/php/src/Play.php b/php/src/Play.php
index 7e90416..a6e758a 100644
--- a/php/src/Play.php
+++ b/php/src/Play.php
@@ -1,13 +1,22 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Theatrical;
 
-class Play 
+class Play
 {
-    public string $name;
-    public string $type;
+    /**
+     * @var string
+     */
+    public $name;
 
-    public function __construct($name, $type)
+    /**
+     * @var string
+     */
+    public $type;
+
+    public function __construct(string $name, string $type)
     {
         $this->name = $name;
         $this->type = $type;
@@ -17,4 +26,4 @@ class Play
     {
         return (string) $this->name . ' : ' . $this->type;
     }
-}
\ No newline at end of file
+}