Install PHP GD image library

PHP Fatal error: Call to undefined function imagefontwidth() 

PHP Fatal error: Call to undefined function imagecreatefrompng()

Debian / Ubuntu :

# apt-get install php5-gd

# /etc/init.d/apache2 restart  

CentOS : 

# yum install php-gd

References :
php - Install GD library and freetype on Linux - Stack Overflow