Why the generated TXT file is empty

Why is the generated txt file empty?
There is no conditional judgment in front of the preceding code, but an empty file is generated

if (!file_exists ("Shang_ci_zong_shu_1.txt")) {//(first build value) if the total number of files does not exist, a TXT file is generated.
$handle =fopen (' Shang_ci_zong_shu_1.txt ', ' W ');
echo "AA"; Test successful output here, but generate empty file
Fwrite ($handle, "1"); Generate default value 1
Fclose ($handle);
}

There is no problem with the IF statement test below:
$handle =fopen (' Shang_ci_zong_shu_1.txt ', ' W ');
Fwrite ($handle, "1"); Generate default value 1
Fclose ($handle);

------Solution--------------------
Consider the return value of fwrite to see what it is.
------Solution--------------------
I can't read it. There's a file for you. Nature will not enter the IF branch.

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