Como decodificar basse 64?

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Ola pessoal gostaria de saber como faço pra decodificar base 64 . . . tentei varios decodificadores e nada e gostaria de aprender passo a passo, lembrando nao to pedindo so pra decodificar eu kero APRENDER a decodificar quem agradeço

 

codigos como estes ai em baixo

<?php

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