preg_replace替换的有关问题

    $string = <<
        
        
        ……………………
str;
    $pattern = array('','','');
    $replace = array('[img]1.jpg[/img]','[img]2.jpg[/img]','[img]3.jpg[/img]');
    echo preg_replace($pattern, $replace, $string);

    …………………… 
    <[img]1.jpg[/img]> 
    <[img]2.jpg[/img]> 
    <[img]3.jpg[/img]> 
    ……………………