Registers a user-defined function for use in SQL statements

# Pdo\Sqlite::createFunction

(PHP 8 >= 8.4.0)

Pdo\Sqlite::createFunction — Registers a user-defined function for use in SQL statements

### 说明

public function Pdo\Sqlite::createFunction(
string `$function_name`,
callable `$callback`,
int `$num_args` = -1,
int `$flags` = 0
): bool

This method allows PHP function to be registered with SQLite as a user-defined function, so that it can be called within SQL queries. The defined function can be used in any SQL query that allows function calls, for example `SELECT`, `UPDATE`, or triggers.

小技巧

By using this method it is possible to override native SQL functions.

### 参数

`function_name`
  The name of the function used in SQL statements.
`callback`
  Callback function to handle the defined SQL function.

  > 注意: Callback functions should return a type understood by SQLite (i.e. scalar type).

  This function need to be defined as:

  function callback(mixed `$value`, mixed `...$values`): mixed
  `value`

    The first argument passed to the SQL function.

  `values`

    Further arguments passed to the SQL function.

`num_args`
  The number of arguments that the SQL function takes. If this parameter is `-1`, then the SQL function may take any number of arguments.
`flags`
  A bitmask of flags. Currently, only `Pdo\Sqlite::DETERMINISTIC` is supported, which specifies that the function always returns the same result given the same inputs within a single SQL statement.

### 返回值

成功时返回 `true`， 或者在失败时返回 `false`。

### 示例

示例 #1 Pdo\Sqlite::createFunction() example

In this example, we have a function that calculates the SHA256 sum of a string, and then reverses it. When the SQL statement executes, it returns the value of the filename transformed by our function. The data returned in $rows contains the processed result.

The beauty of this technique is that there is no need to process the result using a `foreach` loop after the query.

    <?php
    function sha256_and_reverse($string)
    {
        return strrev(hash('sha256', $string));
    }

    $db = new Pdo\Sqlite('sqlite::sqlitedb');
    $db->createFunction('sha256rev', 'sha256_and_reverse', 1);
    $rows = $db->query('SELECT sha256rev(filename) FROM files')->fetchAll();
    ?>

### 参见

- Pdo\Sqlite::createAggregate() - Registers an aggregating user-defined function for use in SQL statements
- Pdo\Sqlite::createCollation() - Registers a user-defined function for use as a collating function in SQL statements
- sqlite_create_function()
- sqlite_create_aggregate()

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