Unable to download composer

I am getting this sort of error while downloading composer.

php composer-setup.php
All settings correct for using Composer
PHP Warning:  file_put_contents(/home/vasu/.composer/keys.dev.pub): failed to open stream: Permission denied in /home/vasu/composer-setup.php on line 468
PHP Warning:  file_put_contents(/home/vasu/.composer/keys.tags.pub): failed to open stream: Permission denied in /home/vasu/composer-setup.php on line 485
Downloading 1.2.0...
PHP Warning:  openssl_verify(): supplied key param cannot be coerced into a public key in /home/vasu/composer-setup.php on line 576
PHP Warning:  openssl_free_key() expects parameter 1 to be resource, boolean given in /home/vasu/composer-setup.php on line 577
Signature mismatch, could not verify the phar file integrity

Easy way

Try the easy way first. The hard way was included in this answer because of the error messages mentioned in the comments.

sudo mkdir -p /usr/local/bin  
sudo apt-get install curl
curl -sS https://getcomposer.org/installer | php   
sudo mv composer.phar /usr/local/bin/composer   

In Ubuntu 16.04 and later composer can be installed from the default Ubuntu repositories. Open the terminal and type:

sudo apt install composer  

Hard way

First check if /usr/local/bin/ exists, and if not make the /usr/local/bin/ directory.

sudo mkdir -p /usr/local/bin

If /usr/local/bin/ already exists, the above command will do nothing except return this message:

mkdir: cannot create directory ‘/usr/local/bin’: File exists

Run these commands:

sudo chmod -R 755 /usr/local/bin/  
cd /usr/local/bin/
ls # list the old composer file(s)
sudo rm <old-composer-files> # delete the old composer file(s)
sudo apt-get install curl
curl -sS https://getcomposer.org/installer | php   
sudo mv composer.phar /usr/local/bin/composer  

Then if you type composer in the terminal it will be accessible globally in your system.