Добавил namespace и зависимости

src/Excel/DataTime.php

@@ -1,6 +1,8 @@
 <?php
 
-class Excel_DateTime
+namespace ctiso\Excel;
+
+class DateTime
 {
     public $value;
 

src/Excel/Document.php

@@ -3,7 +3,11 @@
 /**
  * Документ
  */
-class Excel_Document {
+namespace ctiso\Excel;
+use XMLWriter,
+    Exception;
+
+class Document {
     static  $ns = "urn:schemas-microsoft-com:office:spreadsheet";
     private $table = array ();
     protected $styles = array();

src/Excel/Number.php

@@ -1,6 +1,8 @@
 <?php
 
-class Excel_Number
+namespace ctiso\Excel;
+
+class Number
 {
     public $value;
 

src/Excel/Table.php

@@ -3,6 +3,8 @@
 /**
  * Клетка таблицы 
  */
+namespace ctiso\Excel;
+
 class TableCell 
 {
     public $style = false;
@@ -38,7 +40,7 @@ class TableRow
 /**
  * Таблица 
  */
-class Excel_Table
+class Table
 {
     static    $index;
     private   $name;