<?php
/**
 * Created by PhpStorm.
 * User: a
 * Date: 22.03.18
 * Time: 13:41
 */

$a = '';
$arr = array(
    'str1',
    'str2',
    'str3'
);
foreach ($arr as $value) {
    $a .= $value;
}
echo $a; //выведет str1str2str3