<?php
if (version_compare(phpversion(), "4.3.0", ">=")) {
  // you're on 4.3.0 or later
} else {
  // you're not
}
?>

<?php
//Compare two sets of versions, where major/minor/etc. releases are separated by dots.
//Returns 0 if both are equal, 1 if A > B, and -1 if B < A.
function version_compare2($a, $b)
{
    $a = explode(".", rtrim($a, ".0")); //Split version into pieces and remove trailing .0
    $b = explode(".", rtrim($b, ".0")); //Split version into pieces and remove trailing .0
    foreach ($a as $depth => $aVal)
    { //Iterate over each piece of A
        if (isset($b[$depth]))
        { //If B matches A to this depth, compare the values
            if ($aVal > $b[$depth]) return 1; //Return A > B
            else if ($aVal < $b[$depth]) return -1; //Return B > A
            //An equal result is inconclusive at this point
        }
        else
        { //If B does not match A to this depth, then A comes after B in sort order
            return 1; //so return A > B
        }
    }
    //At this point, we know that to the depth that A and B extend to, they are equivalent.
    //Either the loop ended because A is shorter than B, or both are equal.
    return (count($a) < count($b)) ? -1 : 0;
}
?>

userland_exit("I'm dead");
?>

?>

Should be -1
'1.0.0-pl' vs. '1.1.1-pl'  ---> -1
'1.0.0-pl' vs. ' 1.1.1-pl' ---> 1
    */

However, its quite easy to get working:
    version_compare('1.0.1', '1.0.0pl1', '>')

var_dump ( versionCompare ( '2.9', '<', '2.9.6' ) ); // true
var_dump ( versionCompare ( '2.9', '>', '2.9.6' ) ); // false
var_dump ( versionCompare ( '2.9', '<', '3.0' ) ); // true
var_dump ( versionCompare ( '2.9', '<=', '2.8.9' ) ); // false
var_dump ( versionCompare ( '2.9', '!=', '2.8.9' ) ); // true
var_dump ( versionCompare ( '2.8', '!=', '2.8.0' ) ); // false
var_dump ( versionCompare ( '2.9', '==', '2.9.0' ) ); // true
var_dump ( versionCompare ( '2.9', '==', '2.9.1' ) ); // false
var_dump ( versionCompare ( '3.0 beta', '>', '2.9.9' ) ); // true

Tested on PHP5.3.0, Win32

<?php
version_compare('1.2.3.4RC7.7', '1.2.3.4RC7.8')
version_compare('8.2.50.4', '8.2.52.6')
?>

will both give -1 (ie the left is lower than the right).

<?php
ini_set('error_reporting', (version_compare(PHP_VERSION, '5.3.0', '<') ? E_ALL E_STRICT : E_ALL));
?>

Giving you all the error error reporting you want...

  switch (func_num_args()) {
  case 1: return version_compare($phpversion, $arg1);
  case 2:
    if (preg_match('/^[lg][te] [<>]=? [!=]?= eq ne <>$/i', $arg1))
      return version_compare($phpversion, $arg2, $arg1);
    elseif (preg_match('/^[lg][te] [<>]=? [!=]?= eq ne <>$/i', $arg2))
      return version_compare($phpversion, $arg1, $arg2);
    return version_compare($arg1, $arg2);
  default:
    $ver1 = $arg1;
    if (preg_match('/^[lg][te] [<>]=? [!=]?= eq ne <>$/i', $arg2))
      return version_compare($arg1, $arg3, $arg2);
    return version_compare($arg1, $arg2, $arg3);
  }
}
?>

It also uses phpversion() as a default version if only one string is present. It can make your code look nicer 'cuz you can now write:
<?php if (ver_cmp($version1, '>=', $version2)) something; ?>
and to check a version string against the PHP's version you might use:
<?php if (ver_cmp('>=', $version)) something; ?>
instead of using phpversion().

<?php
function version_compare2($version1, $version2)
{
    $v1 = explode('.',$version1);
    $v2 = explode('.',$version2);
   
    if ($v1[0] > $v2[0])
        $ret = 1;
    else if ($v1[0] < $v2[0])
        $ret = -1;
   
    else    // Major ver are =
    {
        if ($v1[1] > $v2[1])
            $ret = 1;
        else if ($v1[1] < $v2[1])
            $ret = -1;
       
        else  // Minor ver are =
        {
            if ($v1[2] > $v2[2])
                $ret = 1;
            else if ($v1[2] < $v2[2])
                $ret = -1;
            else
                $ret = 0;
        }
    }
   
    return $ret;
}
?>