On the OpenWRT forum I found this:
http://davidoffdotnet.net/openwrt/ipkg/
There are the following packages which provide PHP for the Linksys WRT54g, so they might work on the WL-500g as well:
The first seems to me a simple php library (I don't know what its features are).Code:Package: lwhp Version: 1.0.0 Section: net Architecture: mipsel Maintainer: sam <sam@davidoffdotnet.net> MD5Sum: 96bd50f8625f94f54e62255456b3a683 Size: 143640 Filename: lwhp_1.0.0_mipsel.ipk Source: http://davidoffdotnet.net/openwrt/ip...0.0_mipsel.ipk Description: a lightweight hypertext processor Package: thttpd-php Version: 2.21b-2 Section: net Architecture: mipsel Maintainer: sam <sam@davidoffdotnet.net> MD5Sum: b0660a8072d3c9bfd42a26fccf7571c4 Size: 402230 Filename: thttpd-php_2.21b-2_mipsel.ipk Source: http://davidoffdotnet.net/openwrt/ip...b-2_mipsel.ipk Description: The thttpd webserver compiled with php4 support.
The second package contains a webserver.
I hope this is helpful and, as allways, if it works for you please report back here