Seems (without options) to return a weirdly ordered list of all line numbers in all files where something can be executed.
Like this :
phpdbg_get_executable() =
[<full path 1>] => [
[51] => 0,
[52] => 0,
[53] => 0,
[4] => 0,
[6] => 0,
[7] => 0,
[9] => 0,
[10] => 0],
[<full path 2>] => [
[21] => 0,
[28] => 0,
[29] => 0,
[30] => 0,
[31] => 0,
[32] => 0,
[33] => 0,
[34] => 0,
[35] => 0,
[36] => 0],
etc …