Round DOWN to nearest half integer in PHP

I need a PHP function that will take a float and round it down to the nearest half (x.0 or x.5). I found other functions that will round to the nearest fraction, but they round both ways.

The function I need can only round down.

Examples

7.778 -> 7.5

7.501 -> 7.5

7.49 -> 7.0

7.1 -> 7.0