fgets

A better example, to illustrate the differences in speed for large files, between fgets and stream_get_line.
This example simulates situations where you are reading potentially very long lines, of an uncertain length (but with a maximum buffer size), from an input source.
As Dade pointed out, the previous example I provided was much to easy to pick apart, and did not adequately highlight the issue I was trying to address.
Note that specifying a definitive end-character for fgets (ie: newline), generally decreases the speed difference reasonably significantly. 
#!/usr/bin/php
<?php
 $plaintext=file_get_contents('http://loripsum.net/api/60/verylong/plaintext'); # Should be around 90k characters
 $plaintext=str_replace("\n"," ",$plaintext); # Get rid of newlines
 $fp=fopen("/tmp/SourceFile.txt","w");
 for($i=0;$i<100000;$i++) {
 fputs($fp,substr($plaintext,0,rand(4096,65534)) . "\n");
 }
 fclose($fp);
 $fp=fopen("/tmp/SourceFile.txt","r");
 $start=microtime(true);
 while($line=fgets($fp,65535)) {
 1;
 }
 $end=microtime(true);
 fclose($fp);
 $delta1=($end - $start);
 $fp=fopen("/tmp/SourceFile.txt","r");
 $start=microtime(true);
 while($line=stream_get_line($fp,65535)) {
 1;
 }
 $end=microtime(true);
 fclose($fp);
 $delta2=($end - $start);
 $pdiff=$delta1/$delta2;
 print "stream_get_line is " . ($pdiff>1?"faster":"slower") . " than fgets - pdiff is $pdiff\n";
?>

$ ./testcase.php 
stream_get_line is faster than fgets - pdiff is 1.760398041785
Note that, in a vast majority of situations in which php is employed, tiny differences in speed between system calls are of negligible importance.

if you for some reason need to get lines from a string instead of a file pointer, try
<?php
function string_gets(string $source, int $offset = 0, string $delimiter = "\n"): ?string
{
 $len = strlen($source);
 if ($len < $offset) {
 // out of bounds.. maybe i should throw an exception
 return null;
 }
 if ($len === $offset) {
 // end of string..
 return null;
 }
 $delimiter_pos = strpos($source, $delimiter, $offset);
 if ($delimiter_pos === false) {
 // last line.
 return substr($source, $offset);
 }
 return substr($source, $offset, ($delimiter_pos - $offset) + strlen($delimiter));
}
?>

(i had a ~16GB string in-memory i needed to process line-by-line, but i would get memory-allocation-crash (on a 32GB ram system) if i tried explode("\n",$str); , so came up with this.. interestingly, fgets() seems to be faster than doing it in-ram-in-php, though. php 7.3.7)

There's an error in the documentation:
The file pointer must be valid, and must point to a file successfully opened by fopen() or fsockopen() (and not yet closed by fclose()).
You should also add "popen" and "pclose" to the documentation. I'm a new PHP developer and went to verify that I could use "fgets" on commands that I used with "popen".

fscanf($file, "%s\n") isn't really a good substitution for fgets(), since it will stop parsing at the first whitespace and not at the end of line!
(See the fscanf page for details on this)

I think that the quickest way of read a (long) file with the rows in reverse order is
<?php
$myfile = 'myfile.txt';
$command = "tac $myfile > /tmp/myfilereversed.txt";
passthru($command);
$ic = 0;
$ic_max = 100; // stops after this number of rows
$handle = fopen("/tmp/myfilereversed.txt", "r");
while (!feof($handle) && ++$ic<=$ic_max) {
 $buffer = fgets($handle, 4096);
 echo $buffer."<br>";
}
fclose($handle);
?>

It echos the rows while it is reading the file so it is good for long files like logs.
Borgonovo