In the PHP manual there are many examples which use the cost option in password_hash. Here is some sample code to calculate a good value for cost:
<?php
/**
* This code will benchmark your server to determine how high of a cost you can
* afford. You want to set the highest cost that you can without slowing down
* you server too much. 8-10 is a good baseline, and more is good if your servers
* are fast enough. The code below aims for ≤ 50 milliseconds stretching time,
* which is a good baseline for systems handling interactive logins.
*/
$timeTarget = 0.05; // 50 milliseconds
$cost = 8;
do {
$cost++;
$start = microtime(true);
password_hash("test", PASSWORD_BCRYPT, ["cost" => $cost]);
$end = microtime(true);
} while (($end - $start) < $timeTarget);
echo "Appropriate Cost Found: " . $cost . "\n";
?>
What does cost mean? What is it for?
From wikipedia:
The cost parameter specifies a key expansion iteration count as a power of two, which is an input to the crypt algorithm.