PHP Coding Standards

Always use <?php to delimit PHP code. Do not use the shorthand version, <?. PHP short tags are not enabled by default on new installations and are deprecated.

Use PHP 5 Conventions

Class constructors should use public function __construct() {} rather than the PHP 4 style class name.
Use class destructors where appropriate.
Explicitly declare visibility of member methods and variables (public, private, protected).
Do NOT use closing tags for files containing only PHP code.
The ?> at the end of code files is purposely omitted. This includes for module and include files. ... Removing it eliminates the possibility for unwanted whitespace at the end of files which can cause "header already sent" errors, XHTML/XML validation issues, and other problems.

Indenting and Whitespace

Use indent of 2 spaces, no tabs.
Leave no trailing whitespace at the end of lines.
Files should be formatted with Unix line endings.

Operators

Binary operators (+, =, !=, -, ==, >, etc.) should include a space before and after the operator, for readability. For instance, an assignment would be formatted as $lorem = $ipsum rather than $lorem=$ipsum.
Unary operators (++, --) should not have a space between the operator and the variable or number they are operating on.

Casting

Put a space between the (type) and the $variable in a cast: (int) $mynumber.

Control Structures

Control statements should have one space between the control keyword and opening parenthesis, to distinguish them from function calls.
Always use curly braces even in situations where they are technically optional.
Wrap ternary logic in parens.
Don't use else if, but rather elseif.

Examples

If statements

if (condition1 || condition2) {
  action1;
} elseif (condition3 && condition4) {
  action2;
} else {
  defaultaction;
}

Switch statements

switch (condition) {
  case 1:
    action1;
    break;
case 2:
action2;
break;
default:
defaultaction;
}

For statements

for ($i = 0; $i < condition; $i++) {
  action1;
}

Control Structures in Templates

In templates, a : may be used in place of brackets.
Do no use a space between the closing paren and the colon.
HTML inside the control structure should be indented.

Example:

<?php if (!empty($item)): ?>
  <p><?php print $item; ?></p>
<?php endif; ?>
<?php foreach ($items as $item): ?>
<p><?php print $item; ?></p>
<?php endforeach; ?>

Function Calls

Functions should be called with no spaces between the function name and the opening parenthesis.
No space between the opening paren and the first parameter.
Spaces between commas and each parameter.
No space between the last parameter, the closing parenthesis, and the semicolon
Here's an example: $var = foo($bar, $apple, $peach);

Arrays

A space should separate each element.
Add spaces around the => key association operator.
Example: $arr = array('hey', 'jude', 'git' => 'commit');
If the array spans more than 80 characters, each element should be broken up on its own line.
Multi-line declarations should have elements indented.