function array_interleave($interleave, $array)

libphutil Technical Documentation (Core Utilities)

Add an element between every two elements of some array. That is, given a list A, B, C, D, and some element to interleave, x, this function returns A, x, B, x, C, x, D. This works like implode(), but does not concatenate the list into a string. In particular:

implode('', array_interleave($x, $list));

...is equivalent to:

implode($x, $list);

This function does not preserve keys.