(PHP 4, PHP 5, PHP 7)
  elseif, as its name suggests, is a combination
  of if and else.  Like
  else, it extends an if
  statement to execute a different statement in case the original
  if expression evaluates to
  FALSE.  However, unlike
  else, it will execute that alternative
  expression only if the elseif conditional
  expression evaluates to TRUE.  For example, the
  following code would display a is bigger than
  b, a equal to b
  or a is smaller than b:
  
<?php
if ($a > $b) {
    echo "a is bigger than b";
} elseif ($a == $b) {
    echo "a is equal to b";
} else {
    echo "a is smaller than b";
}
?>
  There may be several elseifs within the same
  if statement.  The first
  elseif expression (if any) that evaluates to
  TRUE would be executed.  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.
 
  The elseif statement is only executed if the
  preceding if expression and any preceding
  elseif expressions evaluated to
  FALSE, and the current
  elseif expression evaluated to
  TRUE.
 
Note: 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.
<?php
/* Incorrect Method: */
if($a > $b):
    echo $a." is greater than ".$b;
else if($a == $b): // Will not compile.
    echo "The above line causes a parse error.";
endif;
/* Correct Method: */
if($a > $b):
    echo $a." is greater than ".$b;
elseif($a == $b): // Note the combination of the words.
    echo $a." equals ".$b;
else:
    echo $a." is neither greater than or equal to ".$b;
endif;
?>