如果之前的项目是用php5.6或者更早版本开发的，现在升级到php7+，那么很多和sql相关的操作代码就要改了，为了同时兼容php的新老版本，就用如下函数即可，参考了discuz的写法
if(!function_exists('mysql_pconnect')){
$mysqli = mysqli_connect("$dbhost:$dbport", $dbuser, $dbpass, $dbname);
function mysql_pconnect($dbhost, $dbuser, $dbpass){
global $dbport;
global $dbname;
global $mysqli;
$mysqli = mysqli_connect("$dbhost:$dbport", $dbuser, $dbpass, $dbname);
return $mysqli;
}
function mysql_select_db($dbname){
global $mysqli;
return mysqli_select_db($mysqli,$dbname);
}
function mysql_fetch_array($result){
return mysqli_fetch_array($result);
}
function mysql_fetch_assoc($result){
return mysqli_fetch_assoc($result);
}
function mysql_fetch_row($result){
return mysqli_fetch_row($result);
}
function mysql_query($query){
global $mysqli;
return mysqli_query($mysqli,$query);
}
function mysql_escape_string($data){
global $mysqli;
return mysqli_real_escape_string($mysqli, $data);
//return addslashes(trim($data));
}
function mysql_real_escape_string($data){
return mysql_real_escape_string($data);
}
function mysql_close(){
global $mysqli;
return mysqli_close($mysqli);
}
}