<?php
if($_SERVER['REQUEST_METHOD'] == 'POST')
{
	$filter1 = strtolower($_POST['select1']);
	if($filter1=="man")
	{
		$ff1	= 'geslacht="man"';
	} elseif($filter1=="vrouw") {
		$ff1	= 'geslacht="vrouw"';
	} else {
		$ff1	= "";
	}

	echo 'select * from table where '.$ff1;
}
?>
<form action="b.php" method="post">
	Filter<br>
	<select name="select1" style="width: 190px">
		<option selected="selected">Selecteer geslacht</option>
		<option>Man</option>
		<option>Vrouw</option>
	</select>
	<input type="submit" name="formSubmit" value="Submit">
</form>