$str = "aaa<br/>bbb<br/>ccc<br/>ddd<br/>";
去除
------解决方案--------------------
$str = "aaa<br/>bbb<br/>ccc<br/>ddd<br/>___";
echo strrev(join('', explode('>/rb<', strrev($str), 2)));
aaa<br/>bbb<br/>ccc<br/>ddd___