Bitwise Operators file function php

Question

Can someone explain me how this function works?

    $size=100 //kb
    if (filesize(file) > ($size << 10))
     echo "file too big";

How does $size << 10 work? How can I make it use MB instead of KB?

Answer 1

The expression $size << 10 shifts the bit pattern to the left 10 times, effectively multiplying by 1024; In other words, it's $size * 1024.

Every time you perform a left shift, you multiply the value by a factor of two. See also bitwise operators.

If you want $size to mean size in MB, you multiply by another 1024, i.e.

if ($filesize($file) > $size * 1024 * 1024) {
    echo "file too big";
}

Or:

if ($filesize($file) > $size << 20) {
    echo "file too big";
}

Answer 2

Numbers are internally represented as binary, a series of zeroes and ones. The << operator will shift all the binary digits left by the specified amount of places, on the right it appends zeroes, example:

  7 << 2
= 111 << 2 (7 = 111 in base two)
= 11100
= 28 (11100 = 28 in base ten)

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The next you need to know that 1024 = 210 and therefore has a 10 digit binary representation, therefore shifting left by 10 digits results in the number being multiplied by 1024.

Answer 3

$size << 10 means $size * pow(2, 10). Tenth power of 2 is 1024, which is number of bytes in a kilobyte. Number of bytes in a megabyte is pow(2, 20); you could write it as $size << 20.

Answer 4

The filesize function returns the size of the file in bytes. The $size value is defined as KB. ($size << 10) converts from KB to bytes so that the comparison is correct.

Answer 5

It's a left shift operator.

100 << 10 means shift 100 to the left by 10. That gives you 102400

It's the same as multiplying by 1024.

It's converting 100kb to bytes, which is what the filesize() returns.