passing information in to php page

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HTML

PHP

I have information in this code

<form action="action_page.php">


First name:<br>
<input type="text" name="firstname" value="Mickey">
<br>
Last name:<br>
<input type="text" name="lastname" value="Mouse">
<br><br>

<input type="radio" name="sex" value="male" checked>Male
<br>
<input type="radio" name="sex" value="female">Female
<br><br>
<input type="checkbox" name="vehicle" value="Bike">I have a bike
<br>
<input type="checkbox" name="vehicle" value="Car">I have a car 
<br><br>

<input type="submit" value="Submit">
</form>

I need code when click on submit send these information to page of php and before this check if any of this field not checked in this case return message like "please fill all options " how can do this