How to read if a checkbox is checked in PHP?

Solution for :How to read if a checkbox is checked in PHP?

If our HTML page looks like this:

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1	<input type="checkbox" name="checkbox1" value="value1">

After submitting the form we can check it with:

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1	isset($_POST['checkbox1'])

OR

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if ($_POST['checkbox1'] == 'value1'){   //I am checked.   }