Use composer for Symfony and its dependencies only, not for eZ Publish and it's dependencies.
For all 5.x versions : Install Zetacomponents for a pure legacy setup
As of eZ Publish 5.2, if you plan to use eZ Publish legacy only, as standalone (as in pointing apache to ezpublish_legacy folder or extracting only ezpublish_legacy), then Zetacomponents must be installed if you don't already have it via PEAR.
After Composer installation (see above for Composer installation instructions) run the following command from the ezpublish_legacy/ folder :
php composer install