<?php
    if(trim(@$_POST['num'])){
        session_start();
        if(isset($_POST['yzm'])){
            if($_SESSION["str"]==$_POST['yzm']){
                echo "right";
                $pdo=new PDO("mysql:host=localhost;dbname=t1","root","");
                $stmt=$pdo->prepare("insert into test(num)values(:num)");
                $stmt->execute(array(':num'=>$_POST['num']);
            }else{
                echo "wrong";
          }
        }
    }
?>

这样写即可