Your problem is that you have quoted the column name. Use
$insert_row = "INSERT INTO comments (comment) VALUES
Author: Nabin Nepal (Starx)
Hello, I am Nabin Nepal and you can call me Starx. This is my blog where write about my life and my involvements. I am a Software Developer, A Cyclist and a Realist. I hope you will find my blog interesting. Follow me on Google+
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