Multiple Database Migration improve performance - php

Question

I have an application where each user gets their own database where each database has the same schema.
I have a script that performs migrations in this fashion:
SHOW databases
Iterate through databases
Execute sql statements
This can take a long time when there are complicated queries that take a lot of time (3 or more seconds)
Is there a way where I can run the sql statements for each database at the same time from one script? Is this dangerous/too resource intensive to do this?
The reason I want to do this, is to prevent downtime as much as possible.
Here is my script now:
<?php
ini_set('display_errors', 1);
set_time_limit(0);
$sql = file_get_contents('../update.sql');
$sql_queries = explode(";", $sql);
$exclude_dbs = array('horde','phppoint_forums','phppoint_site','roundcube', 'pos', 'bntennis_site', 'mysql', 'information_schema', 'performance_schema');
$conn = mysqli_connect("localhost", "root", "PASSWORD");
$show_db_query = mysqli_query($conn, 'SHOW databases');
$databases = array();
while ($row = mysqli_fetch_assoc($show_db_query))
{
if (!in_array($row['Database'], $exclude_dbs))
{
$databases[] = $row['Database'];
}
}
foreach($databases as $database)
{
mysqli_select_db($conn, $database);
echo "Running queries on $database\n***********************************\n";
foreach($sql_queries as $query)
{
if (!empty($query))
{
echo "$query;";
if (!mysqli_query($conn, $query))
{
echo "\n\nERROR: ".mysqli_error($conn)."\n\n";
}
}
}
echo "\n\n";
}
?>

Answer 1

I don't know if the database will hold for that load but basically I would fork the process or spawn it into the background, depending on language.
For php you can fork the process for each database basically running the migrations in parallel.