version_compare() compares two "PHP-standardized" version number strings. This is useful if you would like to write programs working only on some versions of PHP.
version_compare() returns -1 if the first version is lower than the second, 0 if they are equal, and +1 if the second is lower.
The function first replaces _, - and + with a dot . in the version strings and also inserts dots . before and after any non number so that for example '4.3.2RC1' becomes '4.3.2.RC.1'. Then it splits the results like if you were using explode('.', $ver). Then it compares the parts starting from left to right. If a part contains special version strings these are handled in the following order: dev < alpha = a < beta = b < RC < pl. This way not only versions with different levels like '4.1' and '4.1.2' can be compared but also any PHP specific version containing development state.
If you specify the third optional operator argument, you can test for a particular relationship. The possible operators are: <, lt, <=, le, >, gt, >=, ge, ==, =, eq, !=, <>, ne respectively. Using this argument, the function will return TRUE if the relationship is the one specified by the operator, FALSE otherwise.
Note: PHP_VERSION constant holds current PHP version.