I could suggest you
AccuWebHosting.com as the most Reliable, affordable and Resourceful Re-seller Hosting provider with highly optimized web server specially for the PHP websites. Instead of conventional LAMP environment, AccuWebHosting offers Highly optimized Web Hosting environment for PHP websites which includes:
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Now coming to your requirements (20GB - space and 300GB - bandwidth), I could suggest you Linux Reseller Hosting Silver Plan at just $14.25 /month. I am sure you wont find any cheaper Re-seller plan than this.