PHP - Reading Specific Data From A Text File?

$tracking_file_location    =  "../dispatch-manager/logs/tracking_file.txt";
$tracking_file                 =  fopen("$tracking_file_location", "r") or die("Unable to open file!");
$tracking_file_size        =  filesize("$tracking_file_location");
$tracking_file_text        =  fread($tracking_file,$tracking_file_size);
fclose($tracking_file);

I need to read all the rows from a text file between two specific lines into an array where I can process them. I then need to do the same thing again until the end of the file. The file is too large to read the entire file into an array. The number of rows that are between the key lines are variable.

This is what I have so far to read the first set of rows into the array but it is only reading the first row when in fact in this case there are 15. Any pointers as to where I am going wrong please?

if ($_POST['srch']=="Import") {
                            //open file for reading
                            $family_rec=[];
                            $file_handle = fopen($fn, "r");
                            if(!$file_handle){echo "Could not open file";}
                            while(!feof($file_handle)) {
                                $line = fgets($file_handle);
                                if ((strpos($line, "0 @") !== false) && (strpos($line, "FAM") !== false)) {
                                    $famref = explode("@", $line);
                                    $line = fgets($file_handle);
                                    //echo $line."<br>";
                                    do{
                                        $family_rec[]=$line;
                                        $line = fgets($file_handle);
                                        //echo $line."<br>";
                                    }
                                    while((strpos($line, "0 @") !== false) && (strpos($line, "FAM") !== false));
                                    print_r($family_rec);
                                }//end of family

                            }//end of while not eof

                        }//end of import

can someone give me a push here?  I've tried "file_put_contents" and "fputs" and neither one works for me.  I'm running PHP on an IIS server and a windows hosting plan.  PHP version is 7.3.1 I believe.  here is my code:

if ($validation = "true") {
    file_put_contents("ValidationsRedeemed.txt", $validationKey . "|", FILE_APPEND);
    //$handle = fopen("ValidationsRedeemed.txt", "a");
    //fwrite($handle, $validationKey . "|");
    //fclose ($handle);
                            }
else { //user entered a product key that is not recognized.  tell them to try again.
    echo "<script>alert('The product key entered is not valid.'+'\\n\\n'+'Please try again.');
    	 window.location.href='test.php';</script>";
    }

thanks!