Configure xdebug to connect to localhost PHPstorm:

cat /usr/local/etc/php/7.1/conf.d/ext-xdebug.ini:
zend_extension="/usr/local/opt/php71-xdebug/xdebug.so"
xdebug.remote_enable = 1
xdebug.remote_port = 9000
xdebug_remote_server localhost
xdebug.remote_autostart = 1

Set phpstorm to listen. Then execute a simple script:
john@Johns-MacBook-Pro:/Volumes/projects/api/public (master)$ cat test.php
<?php
echo 'hello world';

john@Johns-MacBook-Pro:/Volumes/projects/api/public (master)$ php test.php
Segmentation fault: 11

boom!!

So I looked into this further. Instead of running phpstorm on port 9000 I ran netcat, and discovered that there are no crashes. So then I ran a stack trace and had a look at the traffic with wireshark. Turns out that Xdebug (or PHP) dies horribly during some kind of network 'handshake':

write(0x3, "504\0", 0x1FD) = 509 0 (the big 'hello' XML thing from xdebug)
recvfrom(0x3, 0x7FFF5A40FB00, 0x80) = 37 0
write(0x3, "218\0", 0xDF) = 223 0
recvfrom(0x3, 0x7FFF5A40FB00, 0x80) = 35 0
write(0x3, "216\0", 0xDD) = 221 0
recvfrom(0x3, 0x7FFF5A40FB00, 0x80) = 40 0
write(0x3, "219\0", 0xE0) = 224 0
recvfrom(0x3, 0x7FFF5A40FB00, 0x80) = 12 0
write(0x3, "209\0", 0xD6) = 214 0
recvfrom(0x3, 0x7FFF5A40FB00, 0x80) = 15 0

Most of the other requests I didn't recognise, and I forgot to save the dump. Some of them were showing the pointer to the line in the php file.