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What PHP version do you have? You'll need at least 5.3.7
You might have some issues with permalinks, and PHP might be looking for extensions in the wrong places.
Check if the braille.so binary exists in extension directory on the server OR the extension directory is correctly defined in php.ini. These are the possibilities which is causing problems.
Just comment out the braille line (if you don't need that extension) or fix the directory where it should be, and restart Apache.
And instead of downloading the phar file, I would just install Composer directly from the terminal, so it can make some checks and warn you if there are any issues with your system:
curl -sS https://getcomposer.org/installer | php
Let us know if we can be of any further help!
I am very happy that you're working on converting your modules to XOOPS 2.6.0! That's fantastic!