javascript into if / else

javascript into if / else

I'm not sure if what I wanna do will make any sens to you but here you go.
I'm trying to execute into if / else. Here is an example of what i wanna
do.
<?php
if //Here I would like to execute <script></script>
{
// Then here execute some php code
}
else
{
//do nothing
}
?>
I don't know if such thing is possible. Thank's for your help