bug in implode function ?????

Watch these variables. I dumped them with serialize(). They´re almost identical (the most important part are).I ran implode(',',$var) on them...

array1         a:3:{i:1;i:0;i:2;i:2;i:3;s:5:"ASSOC";}
array2         a:3:{i:1;i:0;i:4;i:1;i:5;s:3:"Sdf";}

string1         ,2,ASSOC
string2         0,1,Sdf

there is the leading zero??? the only difference is, that i created the first array witch func_get_args() and dropped the first element using unset($array1[0]), the second array was created usins $array2=array(1=>0,4=>1,5=>'Sdf'); I´m confused or doesn´t serialize gives a proper picture on how a variable is presented in memory? but even if yes, that does not explain why the leading zero is missing in string1 andré