Enable / Disable textbox controlled by drop-down menu in PHP

<?php
    function dropDown()
    {
        $phpself = $_SERVER['PHP_SELF'];
        echo"<form method='post' action='$phpself'>";
        echo"<select name='categories'>";
        echo"<option value='computer'>computer</option>";
        echo"<option value='mechanical'>mechanical</option>";
        echo"<option value='electronics'>electronics</option>";
        echo"<option value='other'>other</option>";
        echo"</select>";
        echo"<input type='submit' name='submit' value='Submit Form'><br>";
        echo"</form>";
    }
?>
<?php
if(isset($_POST['submit']))
{
    $name = $_POST['categories'];
    if($name == "other"){
        echo "<form >";
        echo "<textarea name='myTextBox' cols='20' rows='1'>";
        echo "Enter some text...";
        echo "</textarea>";
        echo "<br />";
        echo "<input type='submit' ><br>";
        echo "</form>";
    }
    else{
        dropDown();
        return;
    }
}
?>
<?php
    dropDown();
?>