If composer shows proc_open() fork failed on some commands: PHP Fatal error: Uncaught exception 'ErrorException' with message 'proc_open(): fork failed - Cannot allocate memory' in phar This could be happening because the VPS runs out of memory and has no Swap space enabled. free -m total used free shared buffers cached Mem: 2048 357 1690 0 0 237 -/+ buffers/cache: 119 1928 Swap: 0 0 0 To enable the swap you can use for example: $ sudo /bin/dd if=/dev/zero of=/var/swap.1 bs=1M count=1024 $ sudo /sbin/mkswap /var/swap.1 $sudo /sbin/swapon /var/swap.1 Now Install or update composer $ sudo COMPOSER_MEMORY_LIMIT=-1 composer install