En este mensaje hablan de este tema:
https://github.com/PrestaShop/PrestaShop/issues/22300
Aquí te lo comentan:
Problem Solved.
A solution for PHP 7.3from another forum:
There is a problem with constant INTL_IDNA_VARIANT_UTS46 in the file "src/Core/Util/InternationalizedDomainNameConverter.php"
It needs to be modified as follows. The constant INTL_IDNA_VARIANT_UTS46 needs to be converted to int type explicitelly by intval() on the line 47.
FROM:
return $parts[0] . '@' . idn_to_utf8($parts[1], 0, (INTL_IDNA_VARIANT_UTS46));
TO:
return $parts[0] . '@' . idn_to_utf8($parts[1], 0, intval(INTL_IDNA_VARIANT_UTS46));