Decrypted code level 1:

$_F=__FILE__;
$_X='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';
$_X=base64_decode($_X);
$_X=strtr($_X,'123456aouie','aouie123456');
$_X=str_ireplace ('function_','function',$_X);
$_R=preg_replace('/__FILE__/',"'".$_F."'",$_X);
eval($_R);
$_R=0;
$_X=0;

Decrypted code level 2:

$_F='/var/www/malwaredecoder/application/models/Decoder_model.php(664) : eval()'d code';
$_X='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';
eval(base64_decode('JF9YPWJhc2U2NF9kZWNvZGUoJF9YKTskX1g9c3RydHIoJF9YLCcxMjM0NTZhb3VpZScsJ2FvdWllMTIzNDU2Jyk7JF9YPXN0cl9pcmVwbGFjZSAoJ2Z1bmN0aW9uXycsJ2Z1bmN0aW9uJywkX1gpOyRfUj1wcmVnX3JlcGxhY2UoJy9fX0ZJTEVfXy8nLCInIi4kX0YuIiciLCRfWCk7ZXZhbCgkX1IpOyRfUj0wOyRfWD0wOw=='));

Decrypted code level 3:

$_F='/var/www/malwaredecoder/application/models/Decoder_model.php(664) : echo )'d code';
$_X='Pz48P3BocA0KcjVxMzRyNV8ybmM1KCdmY2s1ZDR0MnIucGhwJyk7DQoNCjRmKDRzczV0KCRfUE9TVFsnZnJtX3MzYm00dCddKSl7DQoNCgkkMT0gJF9QT1NUWycxJ107DQoJNWNoMiAkMTsNCn0NCg0KDQoNCj8+DQoNCjxmMnJtIG4xbTU9ImZybSIgMWN0NDJuPSIiIG01dGgyZD0icDJzdCI+DQoJPDRucDN0IHR5cDU9Img0ZGQ1biIgbjFtNT0iZnJtX3MzYm00dCIgdjFsMzU9InMxdjUiIC8+DQoJPD9waHANCgkJCQkkRkNLNWQ0dDJyID0gbjV3IEZDSzVkNHQycignMScpOw0KCQkJCSRGQ0s1ZDR0MnItPkIxczVQMXRoID0gJyc7DQoJCQkJJEZDSzVkNHQyci0+VzRkdGgJPXVpMDsNCgkJCQkkRkNLNWQ0dDJyLT5WMWwzNSA9ICQxOw0KCQkJCSRGQ0s1ZDR0MnItPkNyNTF0NSgpOw0KDQoNCg0KCQkJCSRGQ0s1ZDR0MnI2ID0gbjV3IEZDSzVkNHQycignYicpOw0KCQkJCSRGQ0s1ZDR0MnI2LT5CMXM1UDF0aCA9ICcnOw0KCQkJCSRGQ0s1ZDR0MnI2LT5WMWwzNSA9ICJINCBCIjsNCgkJCQkkRkNLNWQ0dDJyNi0+Q3I1MXQ1KCk7DQoNCgk/Pg0KDQo8L2Yycm0+DQo8MSBocjVmPSJqMXYxc2NyNHB0OiIgMm5jbDRjaz0iZDJjM201bnQuZnJtLnMzYm00dCgpOyI+ICBTM2JtNHQgPC8xPg==';
eval(base64_decode('JF9YPWJhc2U2NF9kZWNvZGUoJF9YKTskX1g9c3RydHIoJF9YLCcxMjM0NTZhb3VpZScsJ2FvdWllMTIzNDU2Jyk7JF9YPXN0cl9pcmVwbGFjZSAoJ2Z1bmN0aW9uXycsJ2Z1bmN0aW9uJywkX1gpOyRfUj1wcmVnX3JlcGxhY2UoJy9fX0ZJTEVfXy8nLCInIi4kX0YuIiciLCRfWCk7ZXZhbCgkX1IpOyRfUj0wOyRfWD0wOw==');

$_F='/var/www/malwaredecoder/application/models/Decoder_model.php(664) : echo )'d code';
$_X='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';
echo base64_decode('JF9YPWJhc2U2NF9kZWNvZGUoJF9YKTskX1g9c3RydHIoJF9YLCcxMjM0NTZhb3VpZScsJ2FvdWllMTIzNDU2Jyk7JF9YPXN0cl9pcmVwbGFjZSAoJ2Z1bmN0aW9uXycsJ2Z1bmN0aW9uJywkX1gpOyRfUj1wcmVnX3JlcGxhY2UoJy9fX0ZJTEVfXy8nLCInIi4kX0YuIiciLCRfWCk7ZXZhbCgkX1IpOyRfUj0wOyRfWD0wOw==';