How to rescrict a JavaScript script being inserted in the database with PHP?

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I have one problem regarding the data insertion in PHP.In my site there is a message system.So when my inbox loads it gives one JavaScript alert.I have searched a lot in my site and finally I found that someone have send me a message with the text below.\[code\]<script> alert(5)</script>\[/code\]So how can I restrict the script code being inserted in my database?I am running on PHP.