php字符串制定字符前添加新的字符

[无涯大者]

如 字符串 <img src="/business/includes/image/20170502/20170502112647_49682.jpg" alt="" /><img src="/business/includes/image/20170502/20170502112648_71811.jpg" alt="" />
想实现给 每个img src 的值前面 添加 http://www.xe123.com 变成

<img src="http://www.xe123.com/business/includes/image/20170502/20170502112647_49682.jpg" alt="" /><img src="http://www.xe123.com/business/includes/image/20170502/20170502112648_71811.jpg" alt="" />

求各位详细指点

[果酱很好吃]

$s = '<img src="/business/includes/image/20170502/20170502112647_49682.jpg" alt="" /><img src="/business/includes/image/20170502/20170502112648_71811.jpg" alt="" />';
$preUrl = 'http://www.xe123.com';
echo preg_replace('#src="/([^"]+?)"#','src="'.$preUrl.'/$1"',$s);