Here is a quick howto upgrade default PHP version provided on Fedora, RHEL or CentOS with latest version 7.4.

Repositories configuration:

On Fedora, standards repositories are enough, on Enterprise Linux (RHEL, CentOS) the Extra Packages for Enterprise Linux (EPEL) repository must be configured, and on RHEL the optional channel must be enabled.

Fedora 31

dnf install https://rpms.remirepo.net/fedora/remi-release-31.rpm

Fedora 30

dnf install https://rpms.remirepo.net/fedora/remi-release-30.rpm

RHEL version 8.1

dnf install https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm
dnf install https://rpms.remirepo.net/enterprise/remi-release-8.rpm

RHEL version 7.7

wget https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
wget https://rpms.remirepo.net/enterprise/remi-release-7.rpm
rpm -Uvh remi-release-7.rpm epel-release-latest-7.noarch.rpm
subscription-manager repos --enable=rhel-7-server-optional-rpms

CentOS version 8.0

dnf install https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm
dnf install https://rpms.remirepo.net/enterprise/remi-release-8.rpm

CentOS version 7.7

wget https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
wget https://rpms.remirepo.net/enterprise/remi-release-7.rpm
rpm -Uvh remi-release-7.rpm epel-release-latest-7.noarch.rpm

php module usage

With Fedora modular and RHEL / CentOS 8, you can simply use the remi-7.4 stream of the php module

dnf module reset php
dnf module install php:remi-7.4

remi-php74 repository activation

Needed packages are in the remi-safe (enabled by default) and remi-php74 repositories, the latest is not enabled by default (administrator choice according to the desired PHP version).

RHEL or CentOS 7

yum install yum-utils
yum-config-manager --enable remi-php74

Fedora

dnf config-manager --set-enabled remi-php74

PHP upgrade

By choice, the packages have the same name than in the distribution, so a simple update is enough:

yum update

That's all :)

$ php -v
PHP 7.4.0 (cli) (built: Nov 26 2019 20:13:36) ( NTS )
Copyright (c) The PHP Group
Zend Engine v3.4.0, Copyright (c) Zend Technologies
    with Zend OPcache v7.4.0, Copyright (c), by Zend Technologies

Known issues

The upgrade can fail (by design) when some installed extensions are not yet compatible with PHP 7.4.

See the compatibility tracking list: PECL extensions RPM status

If these extensions are not mandatory, you can remove them before the upgrade, else, you will have to be patient.

Warning: some extensions are still under development, but it seems useful to provide them to allow upgrade to more people, and to allow user to give feedback to the authors.

More d'information

If you prefer to install PHP 7.4 beside default PHP version, this can be achieved using the php74 prefixed packages, see the PHP 7.4 as Software Collection post.

You can also try the configuration wizard.

The packages available in the repository was used as source for Fedora 32 (self contained change proposal, is already accepted and testable).

By providing a full feature PHP stack, with about 130 available extensions, 6 PHP versions, as base and SCL packages, for Fedora and Enterprise Linux, and with 200 000 download per day, remi repository became in the last 14 years a reference for PHP users on RPM based distributions, maintained by an active contributor to the projects (Fedora, PHP, PECL...).