PHP XSL：键'匹配'模式编译失败

Question

I have the following key but get an apache error when I try to run the code.

<xsl:key name="dt" match="//eprint[substring(date,1,4) &gt;= $year]" use="substring(date,1,4)" />

The error I get is:

[Thu Jul 12 10:51:18.724468 2018] [php7:warn] [pid 24089] [client 127.0.0.1:34808] PHP Warning:  XSLTProcessor::importStylesheet(): Forbidden variable in /home/david/Documents/staffpub/inc/staff.php on line 54, referer: http://localhost:81/test_jsonp.html
[Thu Jul 12 10:51:18.724614 2018] [php7:warn] [pid 24089] [client 127.0.0.1:34808] PHP Warning:  XSLTProcessor::importStylesheet(): compilation error: file /home/david/Documents/staffpub/inc/sorts/year.xsl line 5 element key in /home/david/Documents/staffpub/inc/staff.php on line 54, referer: http://localhost:81/test_jsonp.html
[Thu Jul 12 10:51:18.724634 2018] [php7:warn] [pid 24089] [client 127.0.0.1:34808] PHP Warning:  XSLTProcessor::importStylesheet(): xsl:key : 'match' pattern compilation failed '//eprint[substring(date,1,4) >= $year]' in /home/david/Documents/staffpub/inc/staff.php on line 54, referer: http://localhost:81/test_jsonp.html
[Thu Jul 12 10:51:18.732938 2018] [php7:warn] [pid 24089] [client 127.0.0.1:34808] PHP Warning:  XSLTProcessor::transformToXml(): No stylesheet associated to this object in /home/david/Documents/staffpub/inc/staff.php on line 59, referer: http://localhost:81/test_jsonp.html

I guess there is something wrong with the key set up, but I am unsure what.

Is anyone able to point me in the right direction?

A sample data set is

<?xml version='1.0' encoding='utf-8'?>
<eprints xmlns='http://eprints.org/ep2/data/2.0'>
  <eprint id='http://dsfdsfs'>
   <eprintid>3424243242423</eprintid>
   <date>2012-12</date>
  </eprint>