fgets() returning empty string - php

Question

I am trying to read the first line in a text file, but the fgets() function does not seem to be doing the job. I have a hunch that it might be due to the way the new lines are done in the string, but I am not very experienced in the different ways that a new line can be denoted. I have an image from a website showing what the new lines are inputted as below as well as my code.
link(no rep for images): https://i.gyazo.com/373e217112edbfce272f82b2dae6b317.png
I have already tried changing the mode from w to w+ which I thought would fix the problem, but it did not. I also tried trimming the string using the trim() function in php but that also did not work. I have verified that I am actually writing into the file as well.
Here is the code,
<?php
$input = "def sum(numbers):
total = 0
for x in numbers:
total += x
return total
print(sum((8, 2, 3, 0, 7)))";
$answerFile = fopen("/afs/cad.njit.edu/u/a/j/ajr74/public_html/answer.txt", "w+") or die("Unable to open file.");
fwrite($answerFile, $input);
$line = fgets($answerFile);
print($line);
?>
I am expecting the output to be the first line of the file before the first CR LF tags, but I am getting an empty output instead.

Answer 1

I guess the file pointer is at the end. To bring it back you can use rewind in between writing to your file and reading from it.
So
fwrite($answerFile, $input);
$line = fgets($answerFile);
Becomes
fwrite($answerFile, $input);
rewind($answerFile);
$line = fgets($answerFile);
Note: that's aside from the fact that you already have the content. Normally you wouldn't really need to read form the file, you could just extract the first line from $input. But I guess that you're either just learning or planning to do this in two different points.

Answer 2

Interesting.
After performing a write, you would need to reset the pointer to read the contents back. Best way is to close it and open it for reading.
<?php
$input = "def sum(numbers):
total = 0
for x in numbers:
total += x
return total
print(sum((8, 2, 3, 0, 7)))";
//$file_name = "/afs/cad.njit.edu/u/a/j/ajr74/public_html/answer.txt";
$file_name = "./answer.txt";
echo "Write {$file_name} and Read back a line<br>";
$answerFileHandle = fopen($file_name, "w+") or die("Unable to open file.");
fwrite($answerFileHandle, $input);
// Required these two lines to close and re-open the file.
// Resets the pointer after the write operation above
fclose($answerFileHandle);
$answerFileHandle = fopen($file_name, "r") or die("Unable to open file.");
$line = fgets($answerFileHandle);
echo $line;
fclose($answerFileHandle);
Output is:
`Write ./answer.txt and Read back a line
def sum(numbers):`