Why you should use curly braces in conditionals and loops (code to accompany https://youtu.be/dsu0ooy2hnI)

index.php

<?php

/*
if ( ! file_exists("input.txt"))
    exit;
*/

/*
if ( ! file_exists("input.txt"))
    echo "File not found\n";
    exit;
*/

if ( ! file_exists("input.txt")) {
    echo "File not found\n";
    exit;
}

print "File found ok, open for processing etc.\n";