I have a php script, which can be run independently from the command line, so it's first line is
IMO a script with a shebang is very explicitly a "front end" to the command line and should not be included in other scripts in the first place, because it contains code specific to dealing with CLI interaction (parsing passed arguments and such). It would probably make more sense to separate the reusable code in that script out into a library file, then include that file from both the shebang script and the other script.
I.e., instead of:
#!/usr/bin/php // some code
// some code
#!/usr/bin/php include 'lib.php';