Sorry for reviving this, but if you’re on a Mac, you can also send yourself an email with the desired subject emojis, and when you receive your email, just hit Command-Option-U on your keyborad and copy the subject…
For example:
This Is A Test
comes back as:
=?utf-8?Q?=F0=9F=8E=89_This_Is_A_Test?=
which you can just copy-paste into PHPList
watch out that not all emojis can be read by all… best check compatibility first on a site like https://emojipedia.org
Hope this helps (sorry again for reviving)