This is the code

 <?php

function findSubstringInFile($filePath, $substring) {
    // Open the file in read mode
    $fileHandle = fopen($filePath, "r");
    
    if (!$fileHandle) {
        die("Unable to open file!");
    }

    // Iterate through each line of the file
    while (($line = fgets($fileHandle)) !== false) {
        // Check if the substring is present in the current line
        if (strpos($substring, rtrim($line, "rn")) !== false) {
            // If substring is found, print the line
            echo "Found: " . $line;
            echo "<br>";
        }
    }

    // Close the file handle
    fclose($fileHandle);
}

// Example usage
$filePath = "database.txt";
$substring = file_get_contents("86400.txt");
findSubstringInFile($filePath, $substring);

?>

Here,

1. database.txt is 2.1 GB. So, there are so many lines.
2. Also a 86400.txt file which has 8600 lines data

Aim: I want to get matched line or common lines between database.txt and 86400.txt. My code works well when 86400.txt contains less lines. But for 86400 lines is there any solution?