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# PDO::__construct

(no version information, might be only in CVS)

PDO::__construct -- Creates a PDO instance representing a connection to a database

## 说明

PDO PDO::__construct ( string dsn [, string username [, string password [, array driver_options]]] )

警告
本函数是实验性的。本函数的行为，包括函数名称以及其它任何关于本函数的文档可能会在没有通知的情况下随 PHP 以后的发布而改变。使用本函数风险自担。

Creates a PDO instance to represent a connection to the requested database.

## 参数

dsn

  The Data Source Name, or DSN, contains the information required to connect to the database.

  In general, a DSN consists of the PDO driver name, followed by a colon, followed by the PDO driver-specific connection syntax. Examples of each driver are given below:

  `PDO_DBLIB`

    The DSN prefix is either `sybase:` or `mssql:` depending on which libraries it was linked against during compilation.

    `sybase:host=localhost; dbname=testdb`

    `mssql:host=localhost; dbname=testdb`

  `PDO_FIREBIRD`

    `firebird:User=john;Password=mypass;Database=DATABASE.GDE;DataSource=localhost;Port=3050`

  `PDO_MYSQL`

    `mysql:host=localhost;dbname=testdb`

  `PDO_OCI`

    To connect via `tnsnames.ora`, use:

    `oci:mydb`

    If using instantclient, use:

    `oci:dbname=//localhost:1521/testdb`

  `PDO_ODBC`

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

    DSN=SAMPLE refers to the SAMPLE data source configured in the ODBC driver manager.

  `PDO_PGSQL`

    `pgsql:host=localhost port=5432 dbname=testdb user=john password=mypass`

    Note, by passing user and password in the DSN, the username and password parameters become optional. If specified, they are glued to the end of the connection string.

  `PDO_SQLITE`

    `sqlite:/path/to/database`

    To create a database in memory, use:

    `sqlite::memory:`

  The dsn parameter supports three different methods of specifying the arguments required to create a database connection:

  Driver invocation

    dsn contains the full DSN.

  URI invocation

    dsn consists 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`

  Aliasing

    dsn consists of a name name that maps to pdo.dsn.name in `php.ini` defining the DSN string.

    > 注: The alias must be defined in `php.ini`, and not `.htaccess` or `httpd.conf`

username

  The user name for the DSN string. This parameter is optional for some PDO drivers.

password

  The password for the DSN string. This parameter is optional for some PDO drivers.

driver_options

  A key=>value array of driver-specific connection options.

## 返回值

Returns a PDO object on success.

## 异常

PDO::construct() throws a PDOException if the attempt to connect to the requested database fails.

## 例

例子 1. Create a PDO instance via driver invocation

`<?php // Connect to an ODBC database using driver invocation $dsn = 'mysql:dbname=testdb;host=127.0.0.1'; $user = 'dbuser'; $password = 'dbpass'; try { $dbh = new PDO($dsn, $user, $password); } catch (PDOException $e) { echo 'Connection failed: ' . $e->getMessage(); } ?>`

例子 2. Create a PDO instance via URI invocation

The following example assumes that the file `/usr/local/dbconnect` exists 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 the uri: parameter and pointing to the file URI:

`<?php // Connect to an ODBC database using driver invocation $dsn = 'uri:file:///usr/local/dbconnect'; $user = ''; $password = ''; try { $dbh = new PDO($dsn, $user, $password); } catch (PDOException $e) { echo 'Connection failed: ' . $e->getMessage(); } ?>`

例子 3. Create a PDO instance using an alias

The following example assumes that `php.ini` contains the following entry to enable a connection to a MySQL database using only the alias mydb:

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

`<?php // Connect to an ODBC database using an alias $dsn = 'mydb'; $user = ''; $password = ''; try { $dbh = new PDO($dsn, $user, $password); }catch (PDOException $e) { echo 'Connection failed: ' . $e->getMessage(); } ?>`

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