$content = include "xxx.svg";
preg_match("/<path\s(.*)d=\"(.*)\"/is", $content, $arr);
var_dump($arr[2]);exit;

以下是具体的：

$content = '<?xml version="1.0" encoding="utf-8"?>
<!-- Generator: Adobe Illustrator 16.0.0, SVG Export Plug-In . SVG Version: 6.00 Build 0)  -->
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg version="1.1" id="Layer_1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px"
 width="64px" height="64px" viewBox="0 0 64 64" enable-background="new 0 0 64 64" xml:space="preserve">
<g id="CIRCLE_1_" enable-background="new    ">
<g id="CIRCLE">
<g>
<path d="M32,0C14.327,0,0,14.327,0,32s14.327,32,32,32s32-14.327,32-32S49.673,0,32,0z M32,58C17.641,58,6,46.359,6,32
            C6,17.64,17.641,6,32,6c14.359,0,26,11.64,26,26C58,46.359,46.359,58,32,58z"/>
</g>
</g>';
preg_match("/<path\s(.*)d=\"(.*)\"/is", $content, $arr);
var_dump($arr[2]);
exit;

输出结果：
string 'M32,0C14.327,0,0,14.327,0,32s14.327,32,32,32s32-14.327,32-32S49.673,0,32,0z M32,58C17.641,58,6,46.359,6,32

        C6,17.64,17.641,6,32,6c14.359,0,26,11.64,26,26C58,46.359,46.359,58,32,58z' (length=193)