what's border array keys an array php

i'm minute php book where i

$lines = file('base_list.txt');

to smack record adult an array. record over 100,000 lines it, should 100,000 elements array, nonetheless i run

print_r($lines);
exit;

the array wholly contains 7280 elements.

so i'm curious, wtf? there border volume keys an array have? i'm controlling locally dual-core 2.0ghz 2gb propel (vista & iis though); i'm small confused 4mb record pitch law this.

edit:
i have substantially should have mentioned i formerly set memory_limit 512mb php.ini well.