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.
/** * 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; }