Prepared Statements

The MySQL database supports prepared statements. A prepared statement or a parameterized statement is used to execute the same statement repeatedly with high efficiency and protect against SQL injections.

Basic workflow

The prepared statement execution consists of two stages: prepare and execute. At the prepare stage a statement template is sent to the database server. The server performs a syntax check and initializes server internal resources for later use.

The MySQL server supports using anonymous, positional placeholder with ?.

Prepare is followed by execute. During execute the client binds parameter values and sends them to the server. The server executes the statement with the bound values using the previously created internal resources.

<?php

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
$mysqli = new mysqli("example.com", "user", "password", "database");

/* Non-prepared statement */
$mysqli->query("DROP TABLE IF EXISTS test");
$mysqli->query("CREATE TABLE test(id INT, label TEXT)");

/* Prepared statement, stage 1: prepare */
$stmt = $mysqli->prepare("INSERT INTO test(id, label) VALUES (?, ?)");

/* Prepared statement, stage 2: bind and execute */
$id = 1;
$label = 'PHP';
$stmt->bind_param("is", $id, $label); // "is" means that $id is bound as an integer and $label as a string

$stmt->execute();

Repeated execution

A prepared statement can be executed repeatedly. Upon every execution the current value of the bound variable is evaluated and sent to the server. The statement is not parsed again. The statement template is not transferred to the server again.

<?php

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
$mysqli = new mysqli("example.com", "user", "password", "database");

/* Non-prepared statement */
$mysqli->query("DROP TABLE IF EXISTS test");
$mysqli->query("CREATE TABLE test(id INT, label TEXT)");

/* Prepared statement, stage 1: prepare */
$stmt = $mysqli->prepare("INSERT INTO test(id, label) VALUES (?, ?)");

/* Prepared statement, stage 2: bind and execute */
$stmt->bind_param("is", $id, $label); // "is" means that $id is bound as an integer and $label as a string

$data = [
    1 => 'PHP',
    2 => 'Java',
    3 => 'C++'
];
foreach ($data as $id => $label) {
    $stmt->execute();
}

$result = $mysqli->query('SELECT id, label FROM test');
var_dump($result->fetch_all(MYSQLI_ASSOC));

array(3) {
  [0]=>
  array(2) {
    ["id"]=>
    string(1) "1"
    ["label"]=>
    string(3) "PHP"
  }
  [1]=>
  array(2) {
    ["id"]=>
    string(1) "2"
    ["label"]=>
    string(4) "Java"
  }
  [2]=>
  array(2) {
    ["id"]=>
    string(1) "3"
    ["label"]=>
    string(3) "C++"
  }
}

Every prepared statement occupies server resources. Statements should be closed explicitly immediately after use. If not done explicitly, the statement will be closed when the statement handle is freed by PHP.

Using a prepared statement is not always the most efficient way of executing a statement. A prepared statement executed only once causes more client-server round-trips than a non-prepared statement. This is why the SELECT is not run as a prepared statement above.

Also, consider the use of the MySQL multi-INSERT SQL syntax for INSERTs. For the example, multi-INSERT requires fewer round-trips between the server and client than the prepared statement shown above.

<?php

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
$mysqli = new mysqli("example.com", "user", "password", "database");

$mysqli->query("DROP TABLE IF EXISTS test");
$mysqli->query("CREATE TABLE test(id INT)");

$values = [1, 2, 3, 4];

$stmt = $mysqli->prepare("INSERT INTO test(id) VALUES (?), (?), (?), (?)");
$stmt->bind_param('iiii', ...$values);
$stmt->execute();

Result set values data types

The MySQL Client Server Protocol defines a different data transfer protocol for prepared statements and non-prepared statements. Prepared statements are using the so called binary protocol. The MySQL server sends result set data "as is" in binary format. Results are not serialized into strings before sending. Client libraries receive binary data and try to convert the values into appropriate PHP data types. For example, results from an SQL INT column will be provided as PHP integer variables.

<?php

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
$mysqli = new mysqli("example.com", "user", "password", "database");

/* Non-prepared statement */
$mysqli->query("DROP TABLE IF EXISTS test");
$mysqli->query("CREATE TABLE test(id INT, label TEXT)");
$mysqli->query("INSERT INTO test(id, label) VALUES (1, 'PHP')");

$stmt = $mysqli->prepare("SELECT id, label FROM test WHERE id = 1");
$stmt->execute();
$result = $stmt->get_result();
$row = $result->fetch_assoc();

printf("id = %s (%s)\n", $row['id'], gettype($row['id']));
printf("label = %s (%s)\n", $row['label'], gettype($row['label']));

id = 1 (integer)
label = PHP (string)

This behavior differs from non-prepared statements. By default, non-prepared statements return all results as strings. This default can be changed using a connection option. If the connection option is used, there are no differences.

Fetching results using bound variables

Results from prepared statements can either be retrieved by binding output variables, or by requesting a mysqli_result object.

Output variables must be bound after statement execution. One variable must be bound for every column of the statements result set.

<?php

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
$mysqli = new mysqli("example.com", "user", "password", "database");

/* Non-prepared statement */
$mysqli->query("DROP TABLE IF EXISTS test");
$mysqli->query("CREATE TABLE test(id INT, label TEXT)");
$mysqli->query("INSERT INTO test(id, label) VALUES (1, 'PHP')");

$stmt = $mysqli->prepare("SELECT id, label FROM test WHERE id = 1");
$stmt->execute();

$stmt->bind_result($out_id, $out_label);

while ($stmt->fetch()) {
    printf("id = %s (%s), label = %s (%s)\n", $out_id, gettype($out_id), $out_label, gettype($out_label));
}

id = 1 (integer), label = PHP (string)

Prepared statements return unbuffered result sets by default. The results of the statement are not implicitly fetched and transferred from the server to the client for client-side buffering. The result set takes server resources until all results have been fetched by the client. Thus it is recommended to consume results timely. If a client fails to fetch all results or the client closes the statement before having fetched all data, the data has to be fetched implicitly by mysqli.

It is also possible to buffer the results of a prepared statement using mysqli_stmt::store_result.

Fetching results using mysqli_result interface

Instead of using bound results, results can also be retrieved through the mysqli_result interface. mysqli_stmt::get_result returns a buffered result set.

<?php

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
$mysqli = new mysqli("example.com", "user", "password", "database");

/* Non-prepared statement */
$mysqli->query("DROP TABLE IF EXISTS test");
$mysqli->query("CREATE TABLE test(id INT, label TEXT)");
$mysqli->query("INSERT INTO test(id, label) VALUES (1, 'PHP')");

$stmt = $mysqli->prepare("SELECT id, label FROM test WHERE id = 1");
$stmt->execute();

$result = $stmt->get_result();

var_dump($result->fetch_all(MYSQLI_ASSOC));

array(1) {
  [0]=>
  array(2) {
    ["id"]=>
    int(1)
    ["label"]=>
    string(3) "PHP"
  }
}

Using the mysqli_result interface offers the additional benefit of flexible client-side result set navigation.

<?php

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);
$mysqli = new mysqli("example.com", "user", "password", "database");

/* Non-prepared statement */
$mysqli->query("DROP TABLE IF EXISTS test");
$mysqli->query("CREATE TABLE test(id INT, label TEXT)");
$mysqli->query("INSERT INTO test(id, label) VALUES (1, 'PHP'), (2, 'Java'), (3, 'C++')");

$stmt = $mysqli->prepare("SELECT id, label FROM test");
$stmt->execute();

$result = $stmt->get_result();

for ($row_no = $result->num_rows - 1; $row_no >= 0; $row_no--) {
    $result->data_seek($row_no);
    var_dump($result->fetch_assoc());
}

array(2) {
  ["id"]=>
  int(3)
  ["label"]=>
  string(3) "C++"
}
array(2) {
  ["id"]=>
  int(2)
  ["label"]=>
  string(4) "Java"
}
array(2) {
  ["id"]=>
  int(1)
  ["label"]=>
  string(3) "PHP"
}

Escaping and SQL injection

Bound variables are sent to the server separately from the query and thus cannot interfere with it. The server uses these values directly at the point of execution, after the statement template is parsed. Bound parameters do not need to be escaped as they are never substituted into the query string directly. A hint must be provided to the server for the type of bound variable, to create an appropriate conversion. See the mysqli_stmt::bind_param function for more information.

Such a separation is sometimes considered the only security feature to prevent SQL injection, but the same degree of security can be achieved with non-prepared statements, if all the values are formatted correctly. It should be noted that correct formatting is not the same as escaping and involves more logic than simple escaping. Thus, prepared statements are simply a more convenient and less error-prone approach to this element of database security.

Client-side prepared statement emulation

The API does not include emulation for client-side prepared statement emulation.

See also

• mysqli::__construct
• mysqli::query
• mysqli::prepare
• mysqli_stmt::prepare
• mysqli_stmt::execute
• mysqli_stmt::bind_param
• mysqli_stmt::bind_result