learning CSS is not necessary for webpage design, although your pages are much easier to design (and alter later) if you do.
here is an example of with and without CSS:
without:
<font size="1" color="blue"><b><u><a href="file.html">blah</a></u></b></font>
some other text
<font size="1" color="blue"><b><u><a href="file.html">blah</a></u></b></font>
some other text
<font size="1" color="blue"><b><u><a href="file.html">blah</a></u></b></font>
note that you had to define the style for each and every hyperlink.
with:
<style>
a {
color: blue;
text-decoration: underline;
font-weight: bold;
font-size: 1em;
}
</style>
<a href="file.html">blah</a>
some other text
<a href="file.html">blah</a>
some other text
<a href="file.html">blah</a>
some other text
now, because "a" was configured for a certain style, it is "cascaded" throught the document, instead of having to type it again and again.
remember, this has nothing to do with PHP. PHP just takes data, manipulates it, and returns it to the browser. PHP doesn't care about HTML or CSS or JavaScript.