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;
}