concat() публичный Метод

The arguments can be any valid expression as long as they resolve to strings. If the argument resolves to a value of null or refers to a field that is missing, $concat returns null.
См. также: http://docs.mongodb.org/manual/reference/operator/aggregation/concat/
public concat ( mixed | self $expression1, mixed | self $expression2 )
$expression1 mixed | self
$expression2 mixed | self
Документация по классу Expr
Пример #1
0
 /**
  * Concatenates strings and returns the concatenated string.
  *
  * The arguments can be any valid expression as long as they resolve to
  * strings. If the argument resolves to a value of null or refers to a field
  * that is missing, $concat returns null.
  *
  * @see http://docs.mongodb.org/manual/reference/operator/aggregation/concat/
  * @see Expr::concat
  * @param mixed|Expr $expression1
  * @param mixed|Expr $expression2
  * @param mixed|Expr $expression3,... Additional expressions
  * @return $this
  */
 public function concat($expression1, $expression2)
 {
     $this->expr->concat(...func_get_args());
     return $this;
 }