Below is the list of some code style we agreed upon. These are subjected to change as time goes on.

Inline statements

Inline statements should have the opening and closing PHP tags on the same line.

<?php
// Yes:
<?php echo esc_html($x); ?>

// No:
<?php
echo esc_html($x);
?>

Writing conditionals, or control statements on a single line should be done with braces.

<?php
if ($some_variable === true) { ?>
  <div class="block"></div>
<?php } ?>

Never write inline statements without braces. This is an extremely bad practice because it produces code that is hard to read and maintain.

<?php
// Bad code:
if ($some_variable === true)
  $variable = 'This is inside true statement';

// Worse:
if ($foo) bar();

// Good code:
if ($foo) {
  bar();
}

Multiple statements

Each multiple statement should be on its own line

<?php
// Yes:
$x++;
echo esc_html($x);

// No:
$x++; echo esc_html($x); ?>

Strict comparison

Always use strict comparison inside conditionals, in_array(), array_search(), or any PHP function that has the option to check the value and type of the variable.

It's important to use strict comparison because of type juggling.

If you're not sure what kind of value is returned from the WordPress core function, use WordPress code reference to check.