Differences between DIE/EXIT & INCLUDE/REQUIRE/INCLUDE_ONCE/REQUIRE_ONCE

1 - die & exit

die and exit functions are aliases of eachother and function the same.

2 - how to write: "die();" or "die;" / "exit();" or "exit;"?

<?php
die("Error");

fwrite(STDERR, "Error");
exit(1);
?>

3 - include & require

require is identical to include except upon failure it will also produce a fatal level error (E_COMPILE_ERROR). In other words, it will halt the script whereas include only emits a warning (E_WARNING) which allows the script to continue.

4 - include & include_once // require & require_once

This one is fairly simple, require_once and include_once will only include the file once. For example if you had a for loop like this:

<?php
for ($x = 0; $x <= 10; $x++) {
    include_once('example.php');
    echo $x;
}
?> 

It will only include the file on the first loop.

Where as:

<?php
for ($x = 0; $x <= 10; $x++) {
    include('example.php');
    echo $x;
}
?> 

Will include the file on every loop.

5 - how to write: "include('...');" or "include '...';" // "require('...');" or "require '...';"

Either way is valid syntax, parenthesis are used for grouping.

For example:

<?php
   include('dog.php');
   include 'dog.php';
   include('dog.php') . 'cat.php';
   include 'dog.php' . 'cat.php';
?>