Since php-3.0RC5 it has not been possible to define
constants on the command line. I have the following
script:

[venaas@sheridan /tmp]$ cat phptest
#!/tmp/php -q
<?
        echo $a.":";
?>
[venaas@sheridan /tmp]$ 

With php-3.0RC5 I get

[venaas@sheridan /tmp]$ ./phptest a=5
:[venaas@sheridan /tmp]$ 

With RC4 I get

[venaas@sheridan /tmp]$ ./phptest a=5
5:[venaas@sheridan /tmp]$ 

This was on a Linux box, but the same happens on
Solaris. Both were compiled with

./configure --with-config-file-path=no --enable-debug=no --bindir=/tmp