<input type=“image”>似乎没有发布

我不明白为什么…

这工作:

<?php
    if($_POST['test']) echo "posted";

?>

<form method="POST" action="<?php echo $_SERVER['PHP_SELF']; ?>">
    <input type="submit" name="test" />
</form>

但这并不是:

<?php
    if($_POST['test']) echo "posted";

?>

<form method="POST" action="<?php echo $_SERVER['PHP_SELF']; ?>">
    <input type="image" name="test" src="images/button.submit.png" />
</form>