PDO::__construct

(PHP 5 >= 5.1.0, PHP 7, PHP 8, PECL pdo >= 0.1.0)

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

Description

public PDO::__construct(
    string $dsn,
    ?string $username = null,
    ?string $password = null,
    ?array $options = null
)

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

Parameters

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.

options

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

Errors/Exceptions

PDO::__construct() throws a PDOException if the attempt to connect to the requested database fails, regardless of which PDO::ATTR_ERRMODE is currently set.

Examples

Example #1 Create a PDO instance via driver invocation

<?php
/* Connect to a MySQL database using driver invocation */
$dsn = 'mysql:dbname=testdb;host=127.0.0.1';
$user = 'dbuser';
$password = 'dbpass';

$dbh = new PDO($dsn, $user, $password);

?>

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 = '';

$dbh = new PDO($dsn, $user, $password);

?>

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 = '';

$dbh = new PDO($dsn, $user, $password);

?>

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User Contributed Notes 16 notes

down

124

Kiipa at live dot com ¶

11 years ago


To get UTF-8 charset you can specify that in the DSN.

$link = new PDO("mysql:host=localhost;dbname=DB;charset=UTF8");

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theking2 at king dot ma ¶

1 year ago


A generic pattern to connect to a mariadb or mysql database using a settings file

<?php
$_SETTINGS = parse_ini_file('./settings.ini', true);

$db = new \PDO(
  "mysql:hostname={$_SETTINGS['db']['host']};dbname={$_SETTINGS['db']['name']}",
  $_SETTINGS['db']['user'],
  $_SETTINGS['db']['pass'],
  [
    \PDO::ATTR_DEFAULT_FETCH_MODE => \PDO::FETCH_ASSOC,
    \PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES 'utf8mb4'"
  ]
);
?>
You might not need the options depicted here but I find them convenient. 

Works with a settings.ini file containing for instance:
[db]
host = "localhost"
name = "dbname"
user = "dbuser"
pass = "dbpassword"

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subme at interia dot pl ¶

15 years ago


To specify a database connection port use the following DSN string

<?php
$dsn = 'mysql:dbname=testdb;host=127.0.0.1;port=3333';
?>

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Aymeric S ¶

4 years ago


When trying to connect to a local database, it seems "uri:file:///" is no longer needed. Just write the PDO pilot name followed by the absolute path of the file.
Example : sqlite:C:\VirtualHosts\phpliteadmin\dbs\surveillance_logeas.s3DB

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piotrekkr at o2 dot pl ¶

14 years ago


To connect throught unix socket you need to use 
<?php
$dsn = 'mysql:dbname=testdb;unix_socket=/path/to/socket';
?>

You musn't specify host when using socket.

down

daviddlavier at gmail dot com ¶

9 years ago


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.

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Michal Stefanak ¶

1 year ago


If you override PDO with own class and you want to implement alias from php.ini, you have to get it with `get_cfg_var` instead of `ini_get`.

<?php
class PDO extends \PDO
{
    public function __construct(string $dsn, ?string $username = null, ?string $password = null, ?array $options = null)
    {
        //alias
        if (!str_contains($dsn, ':')) {
            $dsn = get_cfg_var('pdo.dsn.' . $dsn);
            if (!$dsn) {
                throw new PDOException('Argument #1 ($dsn) must be a valid data source name');
            }
        }

        // your additional logic

        parent::__construct($dsn, $username, $password, $options);
    }
}
?>

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Francesco Montanari ¶

4 years ago


Most of the information in the comment here is outdated or wrong.

You do can use host=localhost to connect via socket, which is faster than TCP, so setting 127.0.0.1 is a performance loss.

To use proper utf you should use utf8mb4, for example:

$db = new PDO('mysql:host=' . DATABASE_HOST . ';dbname='. DATABASE_NAME .';charset=utf8mb4', DATABASE_USER, DATABASE_PASSWORD);

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Victor T. ¶

14 years ago


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



<?php

$db = new PDO('mysql:host=myhost;dbname=mydb', 'login', 'password', array(PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES \'UTF8\''));

?>

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max1josef ¶

1 year ago


SQLite3: by default, the database file is created on opening if it does not exist (PDO::SQLITE_OPEN_CREATE).

<?php
$dsn = 'sqlite:D:\Databases\non_existing.db';   // file does not exist

try {
    $dbh = new PDO($dsn);
}
catch(PDOException $e) {
    print $e->getMessage();
}
?> 

No Exception is thrown, instead file "non_existing.db" will be created. To avoid this, use:

<?php
$dsn = 'sqlite:D:\Databases\non_existing.db';
$options = [
    PDO::SQLITE_ATTR_OPEN_FLAGS => PDO::SQLITE_OPEN_READWRITE,  
];

try {
    $dbh = new PDO('sqlite:D:\Databases\non_existing.db', null, null, $options);
}
catch(PDOException $e) {
    print $e->getMessage();
}
?> 
This will ouput:
SQLSTATE[HY000] [14] unable to open database file

If you want to open the database read-only, use PDO::SQLITE_OPEN_READONLY.

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453034559 at qq dot com ¶

2 years ago


// set DEFAULT_FETCH_MODE
<?php
    try {
        $p = new PDO("mysql:host=$db_host;port=$db_port;dbname=$db_dbname", $db_user, $db_pass,array(
            PDO::ATTR_DEFAULT_FETCH_MODE =>PDO::FETCH_ASSOC,
            PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES utf8', //after php5.3.6
        ));
        //$p->exec('SET NAMES utf8');
    } catch (PDOException $e) {
        print "Error!: " . $e->getMessage() . "<br/>";
        die();
    }
?>

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Anonymous ¶

11 years ago


Sqlite:

<?php
try{     
    $pdo = new PDO('sqlite:example.db');
}catch (PDOException $e){
     die ('DB Error');
}
?>

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

down

-1

ohcc at 163 dot com ¶

8 years ago


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.

<?php
    $db = new pdo('mysql:host=127.0.0.1;port=3306;dbname=mysql;charset=utf8','user','password',array(
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
    ));
?>

You should always try ... catch ... 

<?php
    try{
    $db = new pdo('mysql:host=127.0.0.1;port=3306;dbname=mysql;charset=utf8','user','password',array(
        PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
    ));
    }catch(PDOException $pe){
        echo $pe->getMessage();
    }    
?>

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-4

berk0081 at umn dot edu ¶

10 years ago


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



<?php

// The dbname will not be parsed with incorrect casing:

$pdo = new PDO("mysql:host=hostname;DBName=database", "user", "password");



// The correct dbname is lowercase, as displayed in the manual:

$pdo = new PDO("mysql:host=hostname;dbname=database", "user", "password");

?>

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-8

srycroft at excelsystems dot com ¶

8 years ago


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

<?php
   $pdo = new PDO(
     'ibm:' . $host,
     $user,
     $password,
     array(
       PDO::I5_ATTR_DBC_SYS_NAMING => true,
       PDO::I5_ATTR_DBC_LIBL => "QGPL QSYS"
     )
   );
?>

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

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-29

olivernybroe at gmail dot com ¶

8 years ago


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)

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