<?php
// 1.connect mysal
try {
    
    $a; // null
    $b = is_int ( 2 ); // bool
    $c = 2; // int
    $d = 2.5; // float
    
    $str = "abc"; // string;
    $arr = array (
            "username" => "user1",
            "pwd" => 123456 
    ); // array();
    $pdo = new PDO ( "mysql:host=localhost;dbname=stuDemo", "root", "" ); // object
    $file = fopen ( "./data.txt", "w+" ); // resource
    
    
    $dsn = "mysql:host=localhost;dbname=stuDemo";
    $name = "root";
    $pwd = "";
    
    $pdo = new PDO ( $dsn, $name, $pwd ); // object
    
    $pdo -> setAttribute ( PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION ); // bool
} catch ( PDOException $e ) { // $e只是一个变量而已，可以改成$a,$b
    
    $e -> getMessage ();
}

// 2.write sql

$sql = "SELECT * FROM user"; // string
                             
// 3.execute sql
$stmt = $pdo -> query ( $sql ); // object 通过$pdo -> query($sql)方法，得到object(PDOStatement类)
$row = $stmt -> rowCount (); // int
$res = $stmt -> fetch ( PDO::FETCH_ASSOC ); //array $res, result is an array();