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In three minutes, you will learn about the initialization PDO and original SQL statement operations in PHP.

醉折花枝作酒筹
Release： 2023-03-10 19:28:02
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This article will introduce you to the initialization PDO and original SQL statement operations in PHP. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to everyone.

In three minutes, you will learn about the initialization PDO and original SQL statement operations in PHP.

#PDO has become the de facto standard for operating databases in PHP. Including current frameworks and various class libraries, all use PDO as the database connection method. Basically, we only use mysqli to operate the database when writing simple test code or small functions. Note that the normal mysql extension is obsolete!

PDO instance

First let’s take a look at how a PDO instance is initialized. 

$dns = 'mysql:host=localhost;dbname=blog_test;port=3306;charset=utf8';
$pdo = new PDO($dns, 'root', '');
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Under normal circumstances, we can get a PDO object by passing the construction parameters when instantiating it directly. In this way, we have established a connection with the database. If the connection fails, that is, if there is a problem with the parameters written, an exception will be reported directly during instantiation.

The parameters of the PDO object include DNS information, user name, password, and another parameter that can set some properties of the PDO connection. We will see its use later.

dns parameter

The first parameter of the PDO construction parameter is a DNS string. Use semicolons in this string; to separate different parameter contents. The content that can be defined in it includes:

  • DSN prefix, which is the type of database we want to connect to. MySQL databases are generally defined directly using mysql:.

  • host, the connection address, here we are connecting to the local database localhost

  • port, the port number, MySQL defaults to 3306, OK Do not write

  • dbname, the name of the database to be connected is

  • unix_socket, you can specify the MySQL Unix Socket file

  • charset, connection character set

We can use a function to check what database extensions are supported in the current PHP environment: 

print_r(PDO::getAvailableDrivers());exit;
// Array
// (
//     [0] => dblib
//     [1] => mysql
//     [2] => odbc
//     [3] => pgsql
//     [4] => sqlite
// )
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PDO object attributes

The last parameter of the PDO construction parameter can set some attributes of the connection, such as: 

$pdo = new PDO($dns, 'root', '', [PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION]);
showPdoAttribute($pdo);
// ……
// PDO::ATTR_ERRMODE: 2
// ……
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showPdoAttribute() method is our own encapsulation to display all connections Attribute functions. 

// 显示pdo连接属性
function showPdoAttribute($pdo){
    $attributes = array(
        "DRIVER_NAME", "AUTOCOMMIT", "ERRMODE", "CASE", "CLIENT_VERSION", "CONNECTION_STATUS",
        "ORACLE_NULLS", "PERSISTENT", "SERVER_INFO", "SERVER_VERSION"
    );
    
    foreach ($attributes as $val) {
        echo "PDO::ATTR_$val: ";
        echo $pdo->getAttribute(constant("PDO::ATTR_$val")) . "\n";
    }
}
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In this function, we use the getAttribute() method of the PDO instance to obtain the corresponding attribute value. When PDO::ATTR_ERRMODE is not set, its default value is 0, which is the value corresponding to the PDO::ERRMODE_SILENT constant. In the above code, we set it to PDO::ERRMODE_EXCEPTION, and the result of viewing the attribute output becomes 2.

In addition to setting attributes in the parameters of the constructor, we can also use the setAttribute() method of the PDO instance to set the attribute value of PDO. 

pdo2 = new PDO($dns, 'root', '', [PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION]);

echo $pdo2->getAttribute(PDO::ATTR_DEFAULT_FETCH_MODE), PHP_EOL;
// 4

// 设置属性
$pdo2->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC);
echo $pdo2->getAttribute(PDO::ATTR_DEFAULT_FETCH_MODE), PHP_EOL;
// 2
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In this code, we set PDO::ATTR_DEFAULT_FETCH_MODE to PDO::FETCH_ASSOC. In this way, when querying using this $pdo2 connection, the output results will be returned in the form of array key-value pairs. We will immediately enter the study of query-related functions.

Query statement

In most cases, when using PDO, we will use its preprocessing capabilities to write SQL statements. Firstly, the performance is better; secondly, safer. However, let’s not talk about preprocessing issues today. Let’s learn some related functions in the most primitive way of directly operating SQL statements.

Normal query and traversal

// 普通查询 - 遍历1
$stmt = $pdo->query('select * from zyblog_test_user limit 5');
foreach ($stmt as $row) {
    var_dump($row);
}

// array(8) {
//     ["id"]=>
//     string(3) "204"
//     [0]=>
//     string(3) "204"
//     ["username"]=>
//     string(5) "three"
//     [1]=>
//     string(5) "three"
//     ["password"]=>
//     string(6) "123123"
//     [2]=>
//     string(6) "123123"
//     ["salt"]=>
//     string(3) "ccc"
//     [3]=>
//     string(3) "ccc"
//   }
//   ……

// 普通查询 - 遍历2
$stmt = $pdo->query('select * from zyblog_test_user limit 5');

while ($row = $stmt->fetch()) {
    var_dump($row);
}

// array(8) {
//     ["id"]=>
//     string(3) "204"
//     [0]=>
//     string(3) "204"
//     ["username"]=>
//     string(5) "three"
//     [1]=>
//     string(5) "three"
//     ["password"]=>
//     string(6) "123123"
//     [2]=>
//     string(6) "123123"
//     ["salt"]=>
//     string(3) "ccc"
//     [3]=>
//     string(3) "ccc"
//   }
//   ……
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The query() method of a PDO instance executes a query statement and returns a PDOStatement object. By traversing this object, you can obtain the queried data result set.

In the code, we use two methods to traverse, but in fact, their effects are the same. Here, what we want to focus on is the format of the data returned. It can be seen that the data is returned in array format, and in two forms, one is the key name defined by the database, and the other is in subscript form.

Query result set (array, object)

In fact, in most cases, we only need data in the form of key-value pairs of database key names. There are two ways to do this. One is to directly use the $pdo2 connection with the default PDO::ATTR_DEFAULT_FETCH_MODE attribute we defined above, and the other is to specify attributes for the query() method when querying. 

$stmt = $pdo2->query('select * from zyblog_test_user limit 5');
foreach ($stmt as $row) {
    var_dump($row);
}
// array(4) {
//     ["id"]=>
//     string(1) "5"
//     ["username"]=>
//     string(3) "two"
//     ["password"]=>
//     string(6) "123123"
//     ["salt"]=>
//     string(3) "bbb"
//   }
//   ……

$stmt = $pdo->query('select * from zyblog_test_user limit 5', PDO::FETCH_ASSOC);
foreach ($stmt as $row) {
    var_dump($row);
}
// array(4) {
//     ["id"]=>
//     string(1) "5"
//     ["username"]=>
//     string(3) "two"
//     ["password"]=>
//     string(6) "123123"
//     ["salt"]=>
//     string(3) "bbb"
//   }
//   ……
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Of course, we can also directly return the data into the object format. Similarly, we can also use predefined constants to specify query() or the properties of the PDO instance connection. 

$stmt = $pdo->query('select * from zyblog_test_user limit 5', PDO::FETCH_OBJ);
foreach ($stmt as $row) {
    var_dump($row);
}
// object(stdClass)#4 (4) {
//     ["id"]=>
//     string(1) "5"
//     ["username"]=>
//     string(3) "two"
//     ["password"]=>
//     string(6) "123123"
//     ["salt"]=>
//     string(3) "bbb"
//   }
//   ……
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Query result set (class)

The objects in the result set returned in the form of objects above are of type stdClass, which is the default class type of PHP. So can we define a class ourselves and then directly generate its result set after the query is completed? Just like an ORM framework, it completes the mapping of data to objects. Now that I have said that, of course it is possible, just look at the code. 

class user
{
    public $id;
    public $username;
    public $password;
    public $salt;

    public function __construct()
    {
        echo 'func_num_args: ' . func_num_args(), PHP_EOL;
        echo 'func_get_args: ';
        var_dump(func_get_args());
    }
}

class user2
{

}
// 返回指定对象
$u = new user;
$stmt = $pdo->query('select * from zyblog_test_user limit 5', PDO::FETCH_INTO, $u);
foreach ($stmt as $row) {
    var_dump($row);
}
// object(user)#3 (4) {
//     ["id"]=>
//     string(1) "5"
//     ["username"]=>
//     string(3) "two"
//     ["password"]=>
//     string(6) "123123"
//     ["salt"]=>
//     string(3) "bbb"
//   }
//   ……

// 空类测试
$u = new user2;
$stmt = $pdo->query('select * from zyblog_test_user limit 5', PDO::FETCH_INTO, $u);
foreach ($stmt as $row) {
    var_dump($row);
}

// object(user2)#2 (4) {
//     ["id"]=>
//     string(1) "5"
//     ["username"]=>
//     string(3) "two"
//     ["password"]=>
//     string(6) "123123"
//     ["salt"]=>
//     string(3) "bbb"
//   }
//   ……
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在这段代码中，我们定义了两个类，user 类有完整的和数据库字段对应的属性，还定义了一个构造方法（后面会用到）。而 user2 类则是一个空的类。通过测试结果来看，类的属性对于 PDO 来说并不重要。它会默认创建数据库查询到的字段属性，并将它赋值给对象。那么假如我们定义了一个 const 常量属性并给予相同的字段名称呢？大家可以自己尝试一下。

对于 user 和 user2 来说，我们将它实例化了并传递给了 query() ，并且指定了结果集格式为 PDO::FETCH_INTO ，这样就实现了获取对象结果集的能力。但是 PDO 远比你想象的强大，我们还可以直接用类模板来获取查询结果集。

// 根据类返回指定对象
$stmt = $pdo->query('select * from zyblog_test_user limit 5', PDO::FETCH_CLASS, 'user', ['x1', 'x2']);
foreach ($stmt as $row) {
    var_dump($row);
}
// func_num_args: 2
// func_get_args: array(2) {
//   [0]=>
//   string(2) "x1"
//   [1]=>
//   string(2) "x2"
// }
// object(user)#4 (4) {
//   ["id"]=>
//   string(1) "5"
//   ["username"]=>
//   string(3) "two"
//   ["password"]=>
//   string(6) "123123"
//   ["salt"]=>
//   string(3) "bbb"
// }
// ……
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query() 方法直接使用查询结果集模式为 PDO::FETCH_CLASS ，并传递一个类模板的名称，PDO 就会在当前代码中查找有没有对应的类模板，获得的每个结果都会实例化一次。在这里，我们又多了一个参数，最后一个参数是一个数组，并且给了两个元素。估计有不少小伙伴已经看出来了，这个参数是传递给类的构造方法的。记住，使用这个模式，每个元素都会实例化一次，结果集中的每个元素都是新创建的类（object(user2)#3，#号后面的数字是不同的对象句柄id），而 PDO::FETCH_INTO 则是以引用的形式为每个元素赋值（object(user2)#3，#号后面的数字是相同的对象句柄id）。也就是说，我们使用 PDO::FETCH_INTO 模式的时候，修改一个元素的值，其它的元素也会跟着改变，如果使用一个数组去记录遍历的元素值，最后数组的结果也会是相同的最后一个元素的内容。

$stmt = $pdo->query('select * from zyblog_test_user limit 5', PDO::FETCH_INTO, $u);
$resArr = [];
foreach ($stmt as $row) {
    var_dump($row);
    $resArr[] = $row;
}
$resArr[0]->id = 55555;
print_r($resArr);
// Array
// (
//     [0] => user2 Object
//         (
//             [id] => 55555
//             [username] => two
//             [password] => 123123
//             [salt] => bbb
//         )

//     [1] => user2 Object
//         (
//             [id] => 55555
//             [username] => two
//             [password] => 123123
//             [salt] => bbb
//         )

//     [2] => user2 Object
//         (
//             [id] => 55555
//             [username] => two
//             [password] => 123123
//             [salt] => bbb
//         )

//     [3] => user2 Object
//         (
//             [id] => 55555
//             [username] => two
//             [password] => 123123
//             [salt] => bbb
//         )

//     [4] => user2 Object
//         (
//             [id] => 55555
//             [username] => two
//             [password] => 123123
//             [salt] => bbb
//         )

// )
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如何解决这个问题呢？最简单的方式就是在数组赋值的时候加个 clone 关键字呗！

查询结果集（指定字段）

最后轻松一点，我们看下 query() 方法还可以指定查询的某一个字段。

// 只返回第几个字段
$stmt = $pdo->query('select * from zyblog_test_user limit 5', PDO::FETCH_COLUMN, 2);
foreach ($stmt as $row) {
    var_dump($row);
}
// string(32) "bbff8283d0f90625015256b742b0e694"
// string(6) "123123"
// string(6) "123123"
// string(6) "123123"
// string(6) "123123"
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增、删、改操作

除了查询之外的操作，我们也可以使用 exec() 方法来执行其他一些相应的 SQL 语句。

增加操作

$count = $pdo->exec("insert into zyblog_test_user(`username`, `password`, `salt`) value('akk', 'bkk', 'ckk')");
$id = $pdo->lastInsertId();

var_dump($count); // int(1)
var_dump($id); // string(3) "205"
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exec() 返回的是影响的行数，如果我们执行这一条 SQL ，返回的就是成功添加了一行数据。如果要获得新增加数据的 id ，就要使用 lastInserId() 方法来获取。

$count = $pdo->exec("insert into zyblog_test_user(`username`, `password`, `salt`) value('akk', 'bkk', 'ckk', 'dkk')");
// Fatal error: Uncaught PDOException: SQLSTATE[21S01]: Insert value list does not match column list: 1136 Column count doesn't match value count at row 1
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执行错误的 SQL 语句，就像根据 PDO::ATTR_ERRMODE 属性的设置来返回错误信息。我们在最上面的实例化 PDO 代码中指定了错误形式是异常处理模式，所以这里直接就会报 PDOException 异常。

修改操作

// 正常更新
$count = $pdo->exec("update zyblog_test_user set `username`='aakk' where id='{$id}'");

var_dump($count); // int(1)

// 数据不变更新
$count = $pdo->exec("update zyblog_test_user set `username`='aakk' where id='{$id}'");
var_dump($count); // int(0)

// 条件错误更新
$count = $pdo->exec("update zyblog_test_user set `username`='aakk' where id='123123123123'");
var_dump($count); // int(0)
echo '===============', PHP_EOL;
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同样的，在执行更新操作的时候，exec() 返回的也是受影响的行数。很多小伙伴会以这个进行判断是否更新成功，但如果数据没有修改，那么它返回的将是 0 ，SQL 语句的执行是没有问题的，逻辑上其实也没有问题。比如我们在后台打开了某条数据查看，然后并不想更新任何内容就直接点了提交，这时候不应该出现更新失败的提示。也就是说，在前端判断更新操作的时候，需要判断字段是否都有改变，如果没有改变的话那么不应该提示更新失败。这一点是业务逻辑上的考虑问题，如果你认为这样也是更新失败的话，那么这么报错也没有问题，一切以业务形式为主。

删除操作

$count = $pdo->exec("delete from zyblog_test_user where id = '{$id}'");
var_dump($count); // int(1)

// 条件错误删除
$count = $pdo->exec("delete from zyblog_test_user where id = '5555555555'");
var_dump($count); // int(0)
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删除操作需要注意的问题和更新操作是一样的，那就是同样的 exec() 只是返回影响行数的问题，不过相对于更新操作来说，没有受影响的行数那肯定是删除失败的，没有数据被删除。同样的，这个失败的提示也请根据业务情况来具体分析。

总结

不学不知道，一学吓一跳吧，简简单的一个 PDO 的创建和语句执行竟然有这么多的内容。对于我们的日常开发来说，掌握这些原理能够避免很多莫名其妙的问题，比如上面 exec() 只是返回影响行数在业务开发中如何判断操作是否成功的问题就很典型。好了，这只是第一篇，后面的学习不要落下了哦！

测试代码：

https://github.com/zhangyue0503/dev-blog/blob/master/php/202008/source/PHP%E4%B8%AD%E7%9A%84PDO%E6%93%8D%E4%BD%9C%E5%AD%A6%E4%B9%A0%EF%BC%88%E4%B8%80%EF%BC%89%E5%88%9D%E5%A7%8B%E5%8C%96PDO%E5%8F%8A%E5%8E%9F%E5%A7%8BSQL%E8%AF%AD%E5%8F%A5%E6%93%8D%E4%BD%9C.php
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