diff --git a/src/Context.php b/src/Context.php
index cde14a44..87303bd4 100644
--- a/src/Context.php
+++ b/src/Context.php
@@ -28,46 +28,50 @@ class Context
      */
     public $repeat;
 
+    /**
+     * n.b. The following variables *must* be prefixed with an underscore '_' for reasons...
+     */
+
     /**
      * @var string
      */
-    public $xmlDeclaration;
+    public $_xmlDeclaration;
 
     /**
      * @var string
      */
-    public $docType;
+    public $_docType;
 
     /**
      * @var bool
      */
-    private $nothrow;
+    private $_nothrow;
 
 
     /**
      * @var array TODO What type?
      */
-    private $slots = [];
+    private $_slots = [];
 
     /**
      * @var array
      */
-    private $slotsStack = [];
+    private $_slotsStack = [];
 
     /**
      * @var Context
      */
-    private $parentContext;
+    private $_parentContext;
 
     /**
      * @var \stdClass
      */
-    private $globalContext;
+    private $_globalContext;
 
     /**
      * @var bool
      */
-    private $echoDeclarations = false;
+    private $_echoDeclarations = false;
 
     /**
      * Context constructor.
@@ -94,7 +98,7 @@ public function __clone()
      */
     public function setParent(Context $parent): void
     {
-        $this->parentContext = $parent;
+        $this->_parentContext = $parent;
     }
 
     /**
@@ -107,7 +111,7 @@ public function setParent(Context $parent): void
      */
     public function setGlobal(\stdClass $globalContext): void
     {
-        $this->globalContext = $globalContext;
+        $this->_globalContext = $globalContext;
     }
 
     /**
@@ -129,7 +133,7 @@ public function pushContext(): Context
      */
     public function popContext(): Context
     {
-        return $this->parentContext;
+        return $this->_parentContext;
     }
 
     /**
@@ -137,7 +141,7 @@ public function popContext(): Context
      */
     public function echoDeclarations(bool $tf): void
     {
-        $this->echoDeclarations = $tf;
+        $this->_echoDeclarations = $tf;
     }
 
     /**
@@ -156,13 +160,13 @@ public function setDocType(string $doctype, bool $called_from_macro): void
     {
         // FIXME: this is temporary workaround for problem of DOCTYPE disappearing in cloned
         // FIXME: PHPTAL object (because clone keeps _parentContext)
-        if (!$this->docType) {
-            $this->docType = $doctype;
+        if (!$this->_docType) {
+            $this->_docType = $doctype;
         }
 
-        if ($this->parentContext) {
-            $this->parentContext->setDocType($doctype, $called_from_macro);
-        } elseif ($this->echoDeclarations) {
+        if ($this->_parentContext) {
+            $this->_parentContext->setDocType($doctype, $called_from_macro);
+        } elseif ($this->_echoDeclarations) {
             if ($called_from_macro) {
                 throw new Exception\ConfigurationException(
                     'Executed macro in file with DOCTYPE when using echoExecute(). This is not supported yet. ' .
@@ -170,8 +174,8 @@ public function setDocType(string $doctype, bool $called_from_macro): void
                 );
             }
             echo $doctype;
-        } elseif (!$this->docType) {
-            $this->docType = $doctype;
+        } elseif (!$this->_docType) {
+            $this->_docType = $doctype;
         }
     }
 
@@ -191,13 +195,13 @@ public function setDocType(string $doctype, bool $called_from_macro): void
     public function setXmlDeclaration(string $xmldec, bool $called_from_macro): void
     {
         // FIXME
-        if (!$this->xmlDeclaration) {
-            $this->xmlDeclaration = $xmldec;
+        if (!$this->_xmlDeclaration) {
+            $this->_xmlDeclaration = $xmldec;
         }
 
-        if ($this->parentContext) {
-            $this->parentContext->setXmlDeclaration($xmldec, $called_from_macro);
-        } elseif ($this->echoDeclarations) {
+        if ($this->_parentContext) {
+            $this->_parentContext->setXmlDeclaration($xmldec, $called_from_macro);
+        } elseif ($this->_echoDeclarations) {
             if ($called_from_macro) {
                 throw new Exception\ConfigurationException(
                     'Executed macro in file with XML declaration when using echoExecute(). This is not supported yet.' .
@@ -205,8 +209,8 @@ public function setXmlDeclaration(string $xmldec, bool $called_from_macro): void
                 );
             }
             echo $xmldec . "\n";
-        } elseif (!$this->xmlDeclaration) {
-            $this->xmlDeclaration = $xmldec;
+        } elseif (!$this->_xmlDeclaration) {
+            $this->_xmlDeclaration = $xmldec;
         }
     }
 
@@ -220,7 +224,7 @@ public function setXmlDeclaration(string $xmldec, bool $called_from_macro): void
      */
     public function noThrow(bool $bool): void
     {
-        $this->nothrow = $bool;
+        $this->_nothrow = $bool;
     }
 
     /**
@@ -232,7 +236,7 @@ public function noThrow(bool $bool): void
      */
     public function hasSlot(string $key): bool
     {
-        return isset($this->slots[$key]) || ($this->parentContext && $this->parentContext->hasSlot($key));
+        return isset($this->_slots[$key]) || ($this->_parentContext && $this->_parentContext->hasSlot($key));
     }
 
     /**
@@ -246,17 +250,17 @@ public function hasSlot(string $key): bool
      */
     public function getSlot(string $key): string
     {
-        if (isset($this->slots[$key])) {
-            if (is_string($this->slots[$key])) {
-                return $this->slots[$key];
+        if (isset($this->_slots[$key])) {
+            if (is_string($this->_slots[$key])) {
+                return $this->_slots[$key];
             }
             ob_start();
-            call_user_func($this->slots[$key][0], $this->slots[$key][1], $this->slots[$key][2]);
+            call_user_func($this->_slots[$key][0], $this->_slots[$key][1], $this->_slots[$key][2]);
             return ob_get_clean();
         }
 
-        if ($this->parentContext) {
-            return $this->parentContext->getSlot($key);
+        if ($this->_parentContext) {
+            return $this->_parentContext->getSlot($key);
         }
 
         return '';
@@ -273,14 +277,14 @@ public function getSlot(string $key): string
      */
     public function echoSlot(string $key): string
     {
-        if (isset($this->slots[$key])) {
-            if (is_string($this->slots[$key])) {
-                echo $this->slots[$key];
+        if (isset($this->_slots[$key])) {
+            if (is_string($this->_slots[$key])) {
+                echo $this->_slots[$key];
             } else {
-                call_user_func($this->slots[$key][0], $this->slots[$key][1], $this->slots[$key][2]);
+                call_user_func($this->_slots[$key][0], $this->_slots[$key][1], $this->_slots[$key][2]);
             }
-        } elseif ($this->parentContext) {
-            return $this->parentContext->echoSlot($key);
+        } elseif ($this->_parentContext) {
+            return $this->_parentContext->echoSlot($key);
         }
 
         return '';
@@ -295,10 +299,10 @@ public function echoSlot(string $key): string
      */
     public function fillSlot(string $key, string $content): void
     {
-        $this->slots[$key] = $content;
-        if ($this->parentContext) {
+        $this->_slots[$key] = $content;
+        if ($this->_parentContext) {
             // Works around bug with tal:define popping context after fillslot
-            $this->parentContext->slots[$key] = $content;
+            $this->_parentContext->_slots[$key] = $content;
         }
     }
 
@@ -316,10 +320,10 @@ public function fillSlotCallback(
         PhpTalInterface $_thistpl,
         PhpTalInterface $tpl
     ): void {
-        $this->slots[$key] = [$callback, $_thistpl, $tpl];
-        if ($this->parentContext) {
+        $this->_slots[$key] = [$callback, $_thistpl, $tpl];
+        if ($this->_parentContext) {
             // Works around bug with tal:define popping context after fillslot
-            $this->parentContext->slots[$key] = [$callback, $_thistpl, $tpl];
+            $this->_parentContext->_slots[$key] = [$callback, $_thistpl, $tpl];
         }
     }
 
@@ -330,8 +334,8 @@ public function fillSlotCallback(
      */
     public function pushSlots(): void
     {
-        $this->slotsStack[] = $this->slots;
-        $this->slots = [];
+        $this->_slotsStack[] = $this->_slots;
+        $this->_slots = [];
     }
 
     /**
@@ -341,7 +345,7 @@ public function pushSlots(): void
      */
     public function popSlots(): void
     {
-        $this->slots = array_pop($this->slotsStack);
+        $this->_slots = array_pop($this->_slotsStack);
     }
 
     /**
@@ -385,7 +389,7 @@ public function __set(string $varname, $value): void
     public function __isset(string $varname): bool
     {
         // it doesn't need to check isset($this->$varname), because PHP does that _before_ calling __isset()
-        return isset($this->globalContext->$varname) || defined($varname);
+        return isset($this->_globalContext->$varname) || defined($varname);
     }
 
     /**
@@ -402,19 +406,19 @@ public function __get(string $varname)
         // PHP checks public properties first, there's no need to support them here
 
         // must use isset() to allow custom global contexts with __isset()/__get()
-        if (isset($this->globalContext->$varname)) {
-            return $this->globalContext->$varname;
+        if (isset($this->_globalContext->$varname)) {
+            return $this->_globalContext->$varname;
         }
 
         if (defined($varname)) {
             return constant($varname);
         }
 
-        if ($this->nothrow) {
+        if ($this->_nothrow) {
             return null;
         }
 
-        throw new Exception\VariableNotFoundException('Unable to find variable \'$varname\' in current scope');
+        throw new Exception\VariableNotFoundException("Unable to find variable '$varname' in current scope");
     }
 
     /**
diff --git a/src/PHPTAL.php b/src/PHPTAL.php
index 5df85902..a0af1322 100644
--- a/src/PHPTAL.php
+++ b/src/PHPTAL.php
@@ -237,8 +237,8 @@ public function setTemplate(?string $path): PhpTalInterface
         $this->codeFile = null;
         $this->path = $path;
         $this->source = null;
-        $this->context->docType = null;
-        $this->context->xmlDeclaration = null;
+        $this->context->_docType = null;
+        $this->context->_xmlDeclaration = null;
         return $this;
     }
 
@@ -260,8 +260,8 @@ public function setSource(string $src, ?string $path = null): PhpTalInterface
         $this->codeFile = null;
         $this->source = new StringSource($src, $path);
         $this->path = $this->source->getRealPath();
-        $this->context->docType = null;
-        $this->context->xmlDeclaration = null;
+        $this->context->_docType = null;
+        $this->context->_xmlDeclaration = null;
         return $this;
     }
 
@@ -681,13 +681,13 @@ public function execute(): string
             }
 
             // unshift doctype
-            if ($this->context->docType) {
-                $res = $this->context->docType . $res;
+            if ($this->context->_docType) {
+                $res = $this->context->_docType . $res;
             }
 
             // unshift xml declaration
-            if ($this->context->xmlDeclaration) {
-                $res = $this->context->xmlDeclaration . "\n" . $res;
+            if ($this->context->_xmlDeclaration) {
+                $res = $this->context->_xmlDeclaration . "\n" . $res;
             }
 
             if ($this->postfilter !== null) {