Fix test failure by allowing backslash in model name

Author: xhh

modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/PhpClientCodegen.java

@@ -330,7 +330,8 @@ public String toParamName(String name) {
 
     @Override
     public String toModelName(String name) {
-        name = sanitizeName(name);
+        // Note: backslash ("\\") is allowed for e.g. "\\DateTime"
+        name = name.replaceAll("[^\\w\\\\]+", "_");
 
         // model name cannot use reserved keyword
         if (reservedWords.contains(name)) {