Are "elseif" and "else if" completely synonymous?

The Framework Interoperability Group (FIG) which is made up of members that include the developers of Zend ( https://github.com/php-fig/fig-standards#voting-members ) , put together a series of Standard recommendations (PSR-#).

Zend2 and Symfony2 already follows PSR-0.

There's no hard and fast rules for styles, but you can try and follow as much of PSR-2 as you can.

There's a comment on else if vs elseif in PSR-2:

The keyword elseif SHOULD be used instead of else if so that all control keywords look like single words.

https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md#51-if-elseif-else

Some of the recommendations are just that, recommendations. It's up to you whether to use else if or elseif

From the PHP manual:

In PHP, you can also write 'else if' (in two words) and the behavior would be identical to the one of 'elseif' (in a single word). The syntactic meaning is slightly different (if you're familiar with C, this is the same behavior) but the bottom line is that both would result in exactly the same behavior.

Essentially, they will behave the same, but else if is technically equivalent to a nested structure like so:

if (first_condition)
{

}
else
{
  if (second_condition)
  {

  }
}

The manual also notes:

Note that elseif and else if will only be considered exactly the same when using curly brackets as in the above example. When using a colon to define your if/elseif conditions, you must not separate else if into two words, or PHP will fail with a parse error.

Which means that in the normal control structure form (ie. using braces):

if (first_condition)
{

}
elseif (second_condition)
{

}

either elseif or else if can be used. However, if you use the alternate syntax, you must use elseif:

if (first_condition):
  // ...
elseif (second_condition):
  // ...
endif;