form validation

Hello. I want a user to be able to submit only a non-empty form. What is the best way of doing that? Allowed technologies: php, javascript.

[code]
<form action=”submit.php” method=”post”>

message: <input class=s1 type=”text” name=”message” style=”width:50%” value=”<?php if($_SESSION[“i”]>1){echo ”;}else{echo ‘write a new message here..’;} ?>” onfocus=”this.value=”” />

<input class=s1 type=submit value=send>

</form>
[/code]

Could you please help me? Thank you.

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