Site criptografado? É possível?

Olá Pessoal, sou novo no PHP e estou com uma dúvida...

Baixei um site de um cliente para fazer umas alterações só que o site inteiro esta com uns códigos sinistros.

Ex:

(index.php)

<?php // This file is protected by copyright law and provided under license. Reverse engineering of this file is strictly prohibited.
$OOO0O0O00=__FILE__;$O00O00O00=__LINE__;$OO00O0000=13200;eval((base64_decode('JE8wMDBPME8wMD1mb3BlbigkT09PME8wTzAwLCdyYicpO3doaWxlKC0tJE8wME8wME8wMClmZ2V0cygkTzAwME8wTzAwLDEwMjQpO2ZnZXRzKCRPMDAwTzBPMDAsNDA5Nik7JE9PMDBPMDBPMD0oYmFzZTY0X2RlY29kZShzdHJ0cihmcmVhZCgkTzAwME8wTzAwLDg3NiksJ3NDd0xjNDY4ZmpLK0V4QWhrR0IzcFhpT0lRTldaN3RZRC9GUmdNZTJkbjBWbVU1MXVUSlNsUHp2eUhvYmFycTk9JywnQUJDREVGR0hJSktMTU5PUFFSU1RVVldYWVphYmNkZWZnaGlqa2xtbm9wcXJzdHV2d3h5ejAxMjM0NTY3ODkrLycpKSk7ZXZhbCgkT08wME8wME8wKTs=')));return;?>

NiIdKw/nZvxM7wDgB4Gpp4r3GXjiGXjYXg4BpPm2plXBXgXBOlHC3pp2OBgnjFIdfiXJQi7nKwZdKwy0OwynhzGMW8G/QO/lNiHlWvjMZPu5IzrUOwHFZFg2+wGfX4GkOPx4pMQ4pMrikXj3iJ73GXjiGXjY3g4xGB77KBgnY8udK6MSZzXlKwGYplXBXgXBiJ7fX4GkOl/hpPk2OBgnjFIdfiXJQi7nKwZdKwy0OwynhzGMW8G/QO/lNiHlWvjMZPu5IzrUOwHFZFg2+wGYplXBXgXBiJ7fX4GkOl/hpPk2OBgnKBMgNipdjlXS76p

Dá pra fazer alterações? Ou mudar esse arquivo?

Obrigado