According to the PSR2 coding standard:

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

Noncompliant Code Example

if ($expr1) {
  ...
} else if ($expr2) {
  ...
} else {...}

Compliant Solution

if ($expr1) {
  ...
} elseif ($expr2) {
  ...
} else {...}