php create file with given size

You can make use of fopen and fseek

define('SIZE',100); // size of the file to be created.
$fp = fopen('somefile.txt', 'w'); // open in write mode.
fseek($fp, SIZE-1,SEEK_CUR); // seek to SIZE-1
fwrite($fp,'a'); // write a dummy char at SIZE position
fclose($fp); // close the file.

On execution:

$ php a.php

$ wc somefile.txt
  0   1 100 somefile.txt
$ 

If the content of the file is irrelevant then just pad it - but do make sure you don't generate a variable too large to hold in memory:

<?php
$fh = fopen("somefile", 'w');
$size = 1024 * 1024 * 10; // 10mb
$chunk = 1024;
while ($size > 0) {
   fputs($fh, str_pad('', min($chunk,$size)));
   $size -= $chunk;
}
fclose($fh);

If the file has to be readable by something else - then how you do it depends on the other thing which needs to read it.

C.