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# PDO::__construct

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

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

### Descripción

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

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

### Lista de parámetros

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. Further information is available from the PDO driver-specific documentation.

  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.

    > Note: 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.

### Valores retornados

Returns a PDO object on success.

### Errors/Exceptions

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

### Ejemplos

Example #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();
}
?>`

Example #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();
}
?>`

Example #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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