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
