javascript before redirect

[eluser]syntaxerror[/eluser]
hi,
can anyone please tell me how to execute javascript first before redirect?

i have a script here but it skips javascript and executes javascript.

Code:

[removed]
            alert ("Put your warning text here!")
            [removed]
        <?php
            redirect('Front_cont/input_dir_ufpb_wp');
        ?>

i want to check if a file exist,
if that file doesnt exist, it pops up an alert box and return to the
input text page.

the problem is it skips the alert message and just redirect the page.

please advice.

thanks