PHP模板中的换行奇怪

我已经处理这件事很多年了,但它太挑剔了,我从来没有研究过它。但我一直在想。

在PHP模板中,如果我运行此模板:

<p>
<?php echo 'Between "?>" and "</p>" is one linebreak.'; ?>
</p>

<p>
<?php echo 'Between "?>" and "</p>" is one space, then one linebreak.'; ?> 
</p>

<p>
<?php echo 'Between "?>" and "</p>" is two linebreaks.'; ?>

</p>

这是输出:

<p>
Between "?>" and "</p>" is one linebreak.</p>

<p>
Between "?>" and "</p>" is one space, then one linebreak. 
</p>

<p>
Between "?>" and "</p>" is two linebreaks.
</p>

我不理解这种行为。为什么它忽略了一个换行符,就好像它不在那里一样(如第一个例子中)?