How can the PHP script be updated to be compatible with PHP 5.4.21 if it was originally written for PHP 4?

The PHP script needs to be updated to be compatible with PHP 5.4.21 by making necessary changes such as replacing deprecated functions, updating syntax, and ensuring compatibility with newer PHP versions. One common issue with PHP 4 scripts is the use of the "mysql_" functions which are deprecated in PHP 5.4.21 and should be replaced with "mysqli_" functions or PDO for database operations. 

// Original PHP 4 script
mysql_connect("localhost", "username", "password");
mysql_select_db("database");

// Updated PHP 5.4.21 compatible script
$mysqli = new mysqli("localhost", "username", "password", "database");
if ($mysqli->connect_error) {
    die("Connection failed: " . $mysqli->connect_error);
}

Keywords

PHP 5.4.21 PHP script compatibility update migration

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