blocks|key|571637|text|您应该在创建PDO对象时设置数据库。一个例子(来自这里)|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|571638|<?php
$hostname+=+"localhost";
$username+=+"your_username";
$password+=+"your_password";

try+{
++++$dbh+=+new+PDO("mysql:host=$hostname;dbname=mysql",+$username,+$password);
++++echo+"Connected+to+database";+//+check+for+connection
++++}
catch(PDOException+$e)
++++{
++++echo+$e->getMessage();
++++}
?>|code-block|syntax|javascript|571639|entityMap|0|LINK|mutability|MUTABLE|url|http://php.tutorialref.com/pdo-mysql-connect-example.html^0|P|2|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

You should be setting the database when you create the PDO object. An example (from <a href="http://php.tutorialref.com/pdo-mysql-connect-example.html" rel="nofollow">here</a>)

<pre><code>&lt;?php
$hostname = "localhost";
$username = "your_username";
$password = "your_password";

try {
 $dbh = new PDO("mysql:host=$hostname;dbname=mysql", $username, $password);
 echo "Connected to database"; // check for connection
 }
catch(PDOException $e)
 {
 echo $e-&gt;getMessage();
 }
?&gt;
</code></pre>

blocks|key|571645|text|据我所知，您必须为每个连接创建一个新对象。您可以使用连接到多个数据库的方法来扩展PDO类。然后按照你的意愿使用它：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|571646|public+function+pickDatabase($db)+{
++if($db+==+'main')+{
++++return+$this->db['main'];+//instance+of+PDO+object
++else
++++return+$this->db['secondary'];+//another+instance+of+PDO+object
}|code-block|syntax|javascript|571647|并像使用$yourclass->pickDatabase('main')->fetchAll('your+stuff');一样使用它|offset|length|style|CODE|571648|entityMap^0|0|0|4|1L|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@$I|R|J|S|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

As far as I know, you have to create a new object for each connection. You can always extend the PDO class with a method which connects to multiple databases. And then use it as you like:

<pre><code>public function pickDatabase($db) {
 if($db == 'main') {
 return $this-&gt;db['main']; //instance of PDO object
 else
 return $this-&gt;db['secondary']; //another instance of PDO object
}
</code></pre>

and use it like <code>$yourclass-&gt;pickDatabase('main')-&gt;fetchAll('your stuff');</code>

blocks|key|571658|text|通常，在连接时，您会在DSN中指定数据库。但是，如果要创建一个新的数据库，显然不能在创建之前指定该数据库DSN。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|571659|可以使用USE语句更改默认数据库：|offset|length|style|CODE|571660|$dbh+=+new+PDO("mysql:host=...;dbname=mysql",+...);

$dbh->query("create+database+newdatabase");

$dbh->query("use+newdatabase");|code-block|syntax|javascript|571661|随后的CREATE+TABLE语句将在新数据库中创建。|571662|关于@Mike的评论：|571663|当您切换这样的数据库时，它似乎会迫使PDO模拟已准备好的语句。将PDO::ATTR_EMULATE_PREPARES设置为false，然后尝试使用另一个数据库将失败。|blockquote|571664|我只是做了些测试，但我不认为会发生这种事。更改数据库只发生在服务器上，并且它不会改变任何有关PDO在客户端的配置。下面是一个例子：|571665|<?php

//+connect+to+database
try+{
++++$pdo+=+new+PDO('mysql:host=huey;dbname=test',+'root',+'root');
++++$pdo->setAttribute(PDO::ATTR_ERRMODE,+PDO::ERRMODE_EXCEPTION);
++++$pdo->setAttribute(PDO::ATTR_EMULATE_PREPARES,+false);
}+catch(PDOException+$err)+{
++++die($err->getMessage());
}

$stmt+=+$pdo->prepare("select+*+from+foo+WHERE+i+=+:i");
$result+=+$stmt->execute(array("i"=>123));
print_r($stmt->fetchAll(PDO::FETCH_ASSOC));

$pdo->exec("use+test2");

$stmt+=+$pdo->prepare("select+*+from+foo2+WHERE+i+=+:i+AND+i+=+:i");
$result+=+$stmt->execute(array("i"=>456));
print_r($stmt->fetchAll(PDO::FETCH_ASSOC));|571666|如果你说的是真的，那么这应该是没有错误的。只有当PDO::ATTR_EMULATE_PREPARES为真时，PDO才能多次使用给定的命名参数。因此，如果您说这个属性被设置为true，作为更改数据库的副作用，那么它应该可以工作。|571667|但是它不起作用--它得到了一个错误“无效的参数号”，这表明未模拟的准备好的语句仍然有效。|571668|entityMap^0|0|4|3|0|0|3|C|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|13|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|14|8|@$D|15|E|16|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|17|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|18|8|@$D|19|E|1A|F|G]]|9|@]|A|$]]|$1|O|3|P|5|6|7|1B|8|@]|9|@]|A|$]]|$1|Q|3|R|5|S|7|1C|8|@]|9|@]|A|$]]|$1|T|3|U|5|6|7|1D|8|@]|9|@]|A|$]]|$1|V|3|W|5|J|7|1E|8|@]|9|@]|A|$K|L]]|$1|X|3|Y|5|6|7|1F|8|@]|9|@]|A|$]]|$1|Z|3|10|5|6|7|1G|8|@]|9|@]|A|$]]|$1|11|3|-4|5|6|7|1H|8|@]|9|@]|A|$]]]|12|$]]

Typically you would specify the database in the DSN when you connect. But if you're creating a new database, obviously you can't specify that database the DSN before you create it. 

You can change your default database with the <code>USE</code> statement:

<pre><code>$dbh = new PDO("mysql:host=...;dbname=mysql", ...);

$dbh-&gt;query("create database newdatabase");

$dbh-&gt;query("use newdatabase");
</code></pre>

Subsequent <code>CREATE TABLE</code> statements will be created in your newdatabase.

<hr>

Re comment from @Mike:

<blockquote>
 When you switch databases like that it appears to force PDO to emulate prepared statements. Setting PDO::ATTR_EMULATE_PREPARES to false and then trying to use another database will fail.
</blockquote>

I just did some tests and I don't see that happening. Changing the database only happens on the server, and it does not change anything about PDO's configuration in the client. Here's an example:

<pre><code>&lt;?php

// connect to database
try {
 $pdo = new PDO('mysql:host=huey;dbname=test', 'root', 'root');
 $pdo-&gt;setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
 $pdo-&gt;setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
} catch(PDOException $err) {
 die($err-&gt;getMessage());
}

$stmt = $pdo-&gt;prepare("select * from foo WHERE i = :i");
$result = $stmt-&gt;execute(array("i"=&gt;123));
print_r($stmt-&gt;fetchAll(PDO::FETCH_ASSOC));

$pdo-&gt;exec("use test2");

$stmt = $pdo-&gt;prepare("select * from foo2 WHERE i = :i AND i = :i");
$result = $stmt-&gt;execute(array("i"=&gt;456));
print_r($stmt-&gt;fetchAll(PDO::FETCH_ASSOC));
</code></pre>

If what you're saying is true, then this should work without error. PDO can use a given named parameter more than once only if PDO::ATTR_EMULATE_PREPARES is true. So if you're saying that this attribute is set to true as a side effect of changing databases, then it should work. 

But it doesn't work -- it gets an error "Invalid parameter number" which indicates that non-emulated prepared statements remains in effect.

blocks|key|277751|text|或者，您可以在PDO对象已经创建之后选择要使用的MySQL数据库，如下所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|277752|用USE语句。但是请记住，这里使用语句是mysql命令。|277753|try
{
++++$conn+=+new+PDO("mysql:host=$servername;",+$username,+$password);
++++//+set+the+PDO+error+mode+to+exception
++++$conn->setAttribute(PDO::ATTR_ERRMODE,+PDO::ERRMODE_EXCEPTION);

++++$conn->exec("use+databasename");
++++//application+logic
}
catch(PDOException+$e)
{
++++echo+$sql+.+" "+.+$e->getMessage();
}

$conn+=+null;|code-block|syntax|javascript|277754|我希望我的代码对所要求的|277755|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|O|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Alternatively, you can select a MySQL database to use after a PHP PDO object has already been created as below:

With USE STATEMENT. But remember here USE STATEMENT is mysql command

<pre><code>try
{
 $conn = new PDO("mysql:host=$servername;", $username, $password);
 // set the PDO error mode to exception
 $conn-&gt;setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

 $conn-&gt;exec("use databasename");
 //application logic
}
catch(PDOException $e)
{
 echo $sql . "&lt;br&gt;" . $e-&gt;getMessage();
}

$conn = null;
</code></pre>

I hope my code is helpful for requested

I want to select a MySQL database to use after a PHP PDO object has already been created. How do I do this?

<pre><code>// create PDO object and connect to MySQL
$dbh = new PDO( 'mysql:host=localhost;', 'name', 'pass' );

// create a database named 'database_name'

// select the database we just created ( this does not work )
$dbh-&gt;select_db( 'database_name' );
</code></pre>

Is there a PDO equivalent to mysqli::select_db?

Perhaps I'm trying to use PDO improperly? Please help or explain.

EDIT

Should I not be using PDO to create new databases? I understand that the majority of benefits from using PDO are lost on a rarely used operation that does not insert data like <code>CREATE DATABASE</code>, but it seems strange to have to use a different connection to create the database, then create a PDO connection to make other calls.

How do I select a MySQL database to use with PDO in PHP?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我希望在PDO对象已经创建之后选择要使用的MySQL数据库。我该怎么做？// create PDO object and connect to MySQL$dbh = new PDO( 'mysql:host=localhost;', 'name', 'pass' );// create a database name...

问如何选择一个MySQL数据库，以便在PHP中与PDO一起使用？
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问如何选择一个MySQL数据库，以便在PHP中与PDO一起使用？
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