Mcrypt 3DES encrypt/decrypt Help

Hi all: I wan't to use php_mcrpyt to encrypt data,but have a problem in it ! Hint: Warning: mcrypt_encrypt(): Size of key is too large for this algorithm Algorithm : 3DES Key: 48bit <?php // Designate string to be encrypted $string =3D "Applied Cryptography, by Bruce Schneier, is a wonderful cryptography reference."; // Encryption/decryption key $key =3D "C770634F437346D7FC8FA22F9287849E741A33438EEDEAAD"; // Encryption Algorithm $cipher_alg =3D MCRYPT_3DES; // Create the initialization vector for added security. $iv =3D mcrypt_create_iv(mcrypt_get_iv_size($cipher_alg, MCRYPT_MODE_ECB), MCRYPT_RAND); // Output original string print "Original string: $string "; // Encrypt $string $encrypted_string =3D mcrypt_encrypt($cipher_alg, $key, $string, MCRYPT_MODE_CBC, $iv); // Convert to hexadecimal and output to browser print "Encrypted string: ".bin2hex($encrypted_string)." "; $decrypted_string =3D mcrypt_decrypt($cipher_alg, $key, $encrypted_string, MCRYPT_MODE_CBC, $iv); print "Decrypted string: $decrypted_string"; ?>