You will need to downgrade your PHP version temporary, upgrade your prestashop and then upgrade your PHP version again.
Compatible PHP versions
Prestashop 1.6.1.x >> PHP 5.2 - 7.1
Prestashop 1.7.0 ~ 1.7.4 >> PHP 5.4 - 7.1
Prestashop 1.7.5 ~ ... >> PHP 7.2
You will need to downgrade your PHP version temporary, upgrade your prestashop and then upgrade your PHP version again.
Compatible PHP versions
Prestashop 1.6.1.x >> PHP 5.2 - 7.1
Prestashop 1.7.0 ~ 1.7.4 >> PHP 5.4 - 7.1
Prestasgop 1.7.5 ~ ... >> PHP 7.2
