Add note about Concatenation spacing

Author: wilsonge

manual/en-US/coding-standards/chapters/php.md

@@ -145,6 +145,13 @@ $ref1 = &$this->sql;
 >
 > In PHP 5, reference operators are not required for objects. All objects are handled by reference.
 
+## Concatenation Spacing
+There should always be a space between two objects for example
+
+```php
+$variable = 'Hello' . 'World';
+```
+
 ## Arrays
 
 Assignments (the `=>` operator) in arrays may be aligned with tabs. When splitting array definitions onto several lines, the last value may also have a trailing comma. This is valid PHP syntax and helps to keep code diffs minimal.