php正则如何实现全部替换字符串

function getAarr($str){
    //拿出网页中所有a标签放到数组
    $reg1="/<a[\s\S]*?>[\s\S]*?<\/a>/";
    $aarray = array();//这个存放的就是正则匹配出来的所有《a》标签数组
    preg_match_all($reg1,$str,$aarray);
    return $aarray[0];
}
function replace($sou_str,$urls){
    $arr_a = getAarr($sou_str);
    shuffle($urls);
    foreach($arr_a as $i=> $one){
        $begin = strpos($sou_str,$one);//开始位置
        $length = strlen($one);//字符串长度
        if(!isset($urls[$i])){
            $urls[$i]['name'] = str_replace(" ","",strip_tags($one));
            $urls[$i]['url'] = "./?".$urls[$i]['name'];
            $urls[$i] = "<a target='_blank' href='".$urls[$i]['url']."' title='".$urls[$i]['name']."'>".$urls[$i]['name']."</a>";
        }else{
            $urls[$i] = "<a target='_blank' href='".$urls[$i]['url']."' title='".$urls[$i]['name']."'>".$urls[$i]['name']."</a>";
        }
        if ($begin!==false){
            $sou_str = substr_replace($sou_str,$urls[$i],$begin,$length);
        }
    }
    return $sou_str;
}