Acquia recommends using PHP 5.2.x to achieve the fullest results out of your Acquia Drupal website.

Red Hat Enterprise Linux 5 is distributed with PHP 5.1 and some issues exist between some Drupal modules and earlier PHP versions.

Date module: Specifically, on PHP versions prior to 5.2, the Date module is unable to use native date handling. For PHP4, PHP 5.0.x, and PHP 5.1.x, enabling the Date PHP 4 module provides substitutions for some missing date handling. This means core date handling is available, but some issues do exist such as Daylight Savings time and time zones not properly being adjusted. Be cautioned that some date handling functions may not work as expected when using PHP 5.1.

Acquia Search and Apache Solr Search Integration: The Apache Solr Search Integration modules, used by Acquia Search, currently requires PHP 5.2. Support for PHP 5.1 should be available in the near future.