how to delete some lines in a file

if I want to delete some lines in a file, I must fopen it and save it in an array, but I can't delete elements in the array due to lacking of such function, any ideas? if I make it like for ($i=0;i<10;i++)

                   $array[$i]="new";
and write it to the file, the result is file wirte "new"s in one line, not in 10 line saperately. I don't know why.

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