proc_open(): fork failed - Cannot allocate memory

Sometimes, when you run any composer command in Laravel project, you may get error 'proc_open(): fork failed - Cannot allocate memory'. Here is the full error I got when running command composer update:

This could be happening because the VPS runs out of memory and has no Swap space enabled. You can check the current memory and swap using command:

free -m

It will return memory like below: 

You need to enabled swap with following commands:

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

If the above commands not work, then you can try to create a swap file.

sudo fallocate -l 2G /swapfile
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

And now you can use composer command.