You can follow System Requirements for Magento Enterprise Edition and Community Edition.
if you are working with php 5.5, there is no point to downgrade your live environment to 5.3, especially for security reason.
At 1.9.1 index.php
if (version_compare(phpversion(), '5.3.0', '<')===true) {
echo '
<div style="font:12px/1.35em arial, helvetica, sans-serif;">
<div style="margin:0 0 25px 0; border-bottom:1px solid #ccc;">
<h3 style="margin:0; font-size:1.7em; font-weight:normal; text-transform:none; text-align:left; color:#2f2f2f;">Whoops, it looks like you have an invalid PHP version.
</h3>
</div>
<p>Magento supports PHP 5.3.0 or newer.
<a href="http://www.magentocommerce.com/install" target="">Find out</a> how to install</a> Magento using PHP-CGI as a work-around.</p></div>';
exit; }
Note: This question is not about:
why im getting this error on admin page, the question about minimum php version
versioncompare()
function to find the reqd version for that magento.