PHP unable to fully use CPU cores

Hello,
we have a Linode16GB (6CPU) with CentOS 7 64bit.

When doing a PHP script on this Linode (to import products on magento from an XML file) the CPUs are not used at all but system JUST uses 1/6 of the total CPU resources (Load Average 1.0).

I suppose it's a limitation of PHP (it's not multithread by default?).

We expect all 6 CPUs to be fully exploited or almost!

I have made also a PHP script to stress CPU:

<?php
for($i = 0; $i < 1000000000; $i++) {
$a += $i;
}

But even in this case only one CPU is 100%.
If I run the file 6 times the percentage of CPU usage arrives correctly at 600% (100% x6)

Has anyone already had this problem on CentOS?
How can I solve it to use all CPUs with this import?

Thanks a lot to everyone for the support
Kind regards,
Gian Marco.