This is actually the correct behaviour. 
 
implode() can accept its two arguments in any order: the array first, 
or the string. This is possible because it can examine each argument 
to discern which is which based on type. explode() can't do that, 
since it can accept only strings. Essentially, by writing 'explode($a, 
"|")', you are telling PHP to split the string "|" on each occurrence 
of $a within it. To us this is obviously likely to not be what you 
wanted, but to PHP it makes sense--it trusts that this is what you 
want to do, and so it tries it and returns the result: an array 
containing only "|".