No samozrejme, že tie indexy nie sú definované. Tie sa niečím naplnia, len ak na server odošleš nejaký súbor cez <input type="file" name="file"> . To, že si sám spravíš nejakú premennú $file a naplníš ju nejakou hodnotou "text.csv" je úplne irelevantné.
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A. S. Tanenbaum píše:
The terms LF, MF, and HF refer to low, medium, and high frequency, respectively. Clearly, when the names were assigned, nobody expected to go above 10 MHz, so the higher bands were later named the Very, Ultra, Super, Extremely, and Tremendously High Frequency bands. Beyond that there are no names, but Incredibly, Astonishingly, and Prodigiously high frequency (IHF, AHF, and PHF) would sound nice.
