Why is the code of these two readdir loops different?

Could you tell me why the code for these two readdir loops is different?
The previous paragraph can only get a portion of the file
$han 2 = opendir (' dir ');
while (Readdir ($han 2)!== false) {
Echo Readdir ($han 2);
echo "
";
}
Closedir ($han 2);

This section can get all the documents
$od = Opendir (' dir ');
while (($readd = Readdir ($od))!== false) {
Echo $readd;
echo "
";
}
Closedir ($od);
------to solve the idea----------------------
Dir has three files in a B C. The front section while (Readdir ($han 2)!== false) will only move after the cursor is moved to a, and you will be running echo readdir ($han 2) only to read to B. The latter section is used to receive the variable, which can avoid this problem completely.

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