According to code analysis, mcrypt is really optional (fail back to openssl) As RHEL don't provide php-mcrypt, I think it will be cleaner to not require this extension.
php-password-compat-1.0.4-1.fc20 has been submitted as an update for Fedora 20. https://admin.fedoraproject.org/updates/php-password-compat-1.0.4-1.fc20
php-password-compat-1.0.4-1.el6 has been submitted as an update for Fedora EPEL 6. https://admin.fedoraproject.org/updates/php-password-compat-1.0.4-1.el6
php-password-compat-1.0.4-1.fc21 has been submitted as an update for Fedora 21. https://admin.fedoraproject.org/updates/php-password-compat-1.0.4-1.fc21
php-password-compat-1.0.4-1.el7 has been submitted as an update for Fedora EPEL 7. https://admin.fedoraproject.org/updates/php-password-compat-1.0.4-1.el7
Package php-password-compat-1.0.4-1.fc21: * should fix your issue, * was pushed to the Fedora 21 testing repository, * should be available at your local mirror within two days. Update it with: # su -c 'yum update --enablerepo=updates-testing php-password-compat-1.0.4-1.fc21' as soon as you are able to. Please go to the following url: https://admin.fedoraproject.org/updates/FEDORA-2015-0549/php-password-compat-1.0.4-1.fc21 then log in and leave karma (feedback).
php-password-compat-1.0.4-1.fc20 has been pushed to the Fedora 20 stable repository. If problems still persist, please make note of it in this bug report.
php-password-compat-1.0.4-1.fc21 has been pushed to the Fedora 21 stable repository. If problems still persist, please make note of it in this bug report.
php-password-compat-1.0.4-1.el7 has been pushed to the Fedora EPEL 7 stable repository. If problems still persist, please make note of it in this bug report.
php-password-compat-1.0.4-1.el6 has been pushed to the Fedora EPEL 6 stable repository. If problems still persist, please make note of it in this bug report.