PDO::__construct()

(PHP 5 >= 5.1.0, PHP 7, PECL pdo >= 0.1.0)

创建一个表示数据库连接的 PDO 实例

说明

创建一个表示连接到请求数据库的数据库连接 PDO 实例。

参数

数据源名称或叫做 DSN，包含了请求连接到数据库的信息。

通常，一个 DSN 由 PDO 驱动名、紧随其后的冒号、以及具体 PDO 驱动的连接语法组成。更深入的信息能从PDO 具体驱动文档找到。

The$dsn参数支持三种不同的方式创建一个数据库连接：

Driver invocation

$dsn包含完整的DSN。

$dsnconsists of uri: followed by a URI that defines the location of a file containing the DSN string. The URI can specify a local file or a remote URL.

uri:file:///path/to/dsnfile

$dsnconsists of a name$namethat maps topdo.dsn.$nameinphp.inidefining the DSN string.

别名必须得在php.ini中定义了，不能是在.htaccess或httpd.conf中。username

DSN字符串中的用户名。对于某些PDO驱动，此参数为可选项。

DSN字符串中的密码。对于某些PDO驱动，此参数为可选项。

一个具体驱动的连接选项的键=>值数组。

返回值

成功则返回一个PDO对象。

错误／异常

如果试图连接到请求的数据库失败，则PDO::__construct()抛出一个 PDO异常（PDOException）。

范例

Create a PDO instance via driver invocation

Create a PDO instance via URI invocation

The following example assumes that the file/usr/local/dbconnectexists with file permissions that enable PHP to read the file. The file contains the PDO DSN to connect to a DB2 database through the PDO_ODBC driver:

odbc:DSN=SAMPLE;UID=john;PWD=mypass

The PHP script can then create a database connection by simply passing theuri:parameter and pointing to the file URI:

使用别名创建一个PDO实例

The following example assumes thatphp.inicontains the following entry to enable a connection to a MySQL database using only the aliasmydb:

[PDO]
pdo.dsn.mydb="mysql:dbname=testdb;host=localhost"

To get UTF-8 charset you can specify that in the DSN.
$link = new PDO("mysql:host=localhost;dbname=DB;charset=UTF8");

I'd like to point out that in PHP 7.0 in the dsn parameter you can't use 'host=localhost' to solve this you can use 'host=127.0.0.1' instead.

To connect throught unix socket you need to use 

You musn't specify host when using socket.

To specify a database connection port use the following DSN string 

If you use the UTF-8 encoding, you have to use the fourth parameter : 

Sqlite:

If 'example.db' does not exist, no exception is thrown but the file 'example.db' is created.

Although not explicitly stated, parameters in the PDO $dsn string may be case-sensitive on some platforms or drivers. 

You will get a fatal error if you don't catch the exception threw by PDO when it fails to connect to the database server like this.
Fatal error: in xxx.php on line xx
This error neither can be handled by error handlers nor can it be erased by the @ sign, which can make your script uncontrollable.

You should always try ... catch ... 

For IBM i / iSeries / AS400 DB2 connections, you can specify system naming and library lists for php_pdo modules > 1.3.3-sg2

See http://yips.idevcloud.com/wiki/index.php/XMLSERVICE/PHPPDOChangeLog

Just to give an odbc string connection example to MSSQL:
$odbc="odbc:Driver={SQL Server};Server=$server;Database=$database;";
$cnx = new PDO( $odbc , $user, $password);
$sql="SELECT field FROM Quotation_Header WHERE (field> 0)";
foreach ($cnx->query($sql) as $row) {
  echo $row['field'] . "\n";
}

To connect to the database via a function use this and call the getConnection function in the class constructor. 

If you happen to create a dsn like this...

... your result will be bool(false) and your $db->errorInfo() will be "0000" which isn't very helpful.
You have to look to the $stmt->errorInfo() to see "No database selected" which is even more confusing until you realize that it's not 'database' but 'dbname' in your DSN! You would think the error about a bad DSN would be in the $db errorInfo but it is not, it's not until you try the $stmt that the error is generated.

I wasted a half hour on that one.

The PDO connection is case-sensitive, this means that you cannot write 
`$PDO = new PDO("MySQL:DBName=dbname;host=localhost");`
You would have to write it 
`$PDO = new PDO("mysql:dbname=dbname;host=localhost");`
The difference here is that `mysql` and `dbname` is with all lower-case.
Some IDE's like PHPStorm will show a `TYPO ERROR`, at `dbname` if it's written with lower-case only, this is just to be ignored and have been reported to PHPStorm for them to fix. (Currrent version 10.0.2)

pdo单例模式

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