http://www.zend.com/store/products/zend-platform/system-requirements.php
Supported Operating Systems, Platforms and OS versions:
# Linux x86
# Linux x86-64
# Solaris Sparc 8, 9, 10
# Solaris x86 9, 10
# FreeBSD x86 5.4, 5.5
PHP Compatibility for version 2.1:
# Supported PHP versions: 4.3.9 and above, 4.4.x, 5.0.x, 5.1 RC2