how to monitor and display most searched item

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HTML

PHP

I am building a website where a user can
search for items. I want to monitor and display
most searched items on the website. I want to
create a trend of what people are searching
for using the website (e.g like yahoo trends). I
have been trying to figure out how to
implement this but I am clueless. I have
searched the internet but it seems I am
missing the specific keywords.
I will like ideas, short tutorials or snippets on
how to achieve this, thanks.