PHPCS: fix up the code base [1] - whitespace & inline control structures

[jrfnl]

Simple, mostly whitespace related, fixes to make the code comply with the WPCliCS rules.

Fixes relate to the following rules:

• Use single quoted strings when the string doesn't contain a single quote or variables.
• Align the equality operators in assignment blocks.
• No space between else and if for an elseif.
• One space on the inside of open/close parenthesis when there is content between them.
• Associative arrays with more than one item, should be multi-line.
• The double arrow in multi-line arrays should be aligned.
• Each array item in a multi-line array should be followed by a comma.
• When using array indexes, use no space on the inside of the square brackets when the index is a string or integer and use one space on the inside of the square brackets otherwise.
• Use tabs for indentation, spaces for inline alignment.
• No precision alignment.

[schlessera]

I'm sneaking in an additional commit into this one (to avoid further code churn) to make strings even more consistent:

• Avoid multiple string concatenation and always prefer variable substitution
• Always enclose variable substitution in curly braces