Concepts
These concepts are specific to the MySQL drivers for PHP.
Buffered and Unbuffered queries
Queries are using the buffered mode by default. This means that query results are
immediately transferred from the MySQL Server to PHP in is then kept in the memory
of the PHP process. This allows additional operations like counting the
number of rows, and moving (seeking) the current result pointer. It also allows
issuing further queries on the same connection while working on the result set.
The downside of the buffered mode is that larger result sets might require
quite a lot memory. The memory will be kept occupied till all references to the
result set are unset or the result set was explicitly freed, which will automatically
happen during request end the latest. The terminology "store result" is also used
for buffered mode, as the whole result set is stored at once.
When using libmysql as library PHP's memory limit won't count the memory used
for result sets unless the data is fetched into PHP variables. With mysqlnd
the memory accounted for will include the full result set.
Unbuffered MySQL queries execute the query and then return a resource while
the data is still waiting on the MySQL server for being fetched. This uses less memory
on the PHP-side, but can increase the load on the server. Unless the full result set was
fetched from the server no further queries can be sent over the same connection. Unbuffered
queries can also be referred to as "use result".
Following these characteristics buffered queries should be used in cases where you expect
only a limited result set or need to know the amount of returned rows before reading all
rows. Unbuffered mode should be used when you expect larger results.
Because buffered queries are the default, the examples below will demonstrate how to
execute unbuffered queries with each API.
Unbuffered query example: mysqli
query("SELECT Name FROM City", MYSQLI_USE_RESULT);
if ($uresult) {
while ($row = $uresult->fetch_assoc()) {
echo $row['Name'] . PHP_EOL;
}
}
$uresult->close();
?>
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Unbuffered query example: pdo_mysql
setAttribute(PDO::MYSQL_ATTR_USE_BUFFERED_QUERY, false);
$uresult = $pdo->query("SELECT Name FROM City");
if ($uresult) {
while ($row = $uresult->fetch(PDO::FETCH_ASSOC)) {
echo $row['Name'] . PHP_EOL;
}
}
?>
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Unbuffered query example: mysql
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Character sets
Ideally a proper character set will be set at the server level, and doing this is described
within the Character Set Configuration
section of the MySQL Server manual. Alternatively, each MySQL API offers a method to set
the character set at runtime.
The character set and character escaping
The character set should be understood and defined, as it has an affect on every
action, and includes security implications. For example, the escaping mechanism
(e.g., mysqli_real_escape_string for mysqli, mysql_real_escape_string
for mysql, and PDO::quote for PDO_MySQL) will adhere to
this setting. It is important to realize that these functions will not use the character
set that is defined with a query, so for example the following will not have an effect
on them:
Problems with setting the character set with SQL
real_escape_string();
$mysqli->query("SET NAMES utf8");
// Will not affect $mysqli->real_escape_string();
$mysqli->query("SET CHARACTER SET utf8");
// But, this will affect $mysqli->real_escape_string();
$mysqli->set_charset('utf8');
?>
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Below are examples that demonstrate how to properly alter the character set at runtime
using each each API.
Setting the character set example: mysqli
set_charset('utf8')) {
printf("Error loading character set utf8: %s\n", $mysqli->error);
} else {
printf("Current character set: %s\n", $mysqli->character_set_name());
}
print_r( $mysqli->get_charset() );
?>
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Setting the character set example: <link linkend="ref.pdo-mysql.connection">pdo_mysql</link>
Note: This only works as of PHP 5.3.6.
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Setting the character set example: mysql
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