version_compare

[editors note]
snipbit fixed after comment from Matt Mullenweg
--jm
[/editors note]
so in a nutshell... I believe it works best like this:
<?php
if (version_compare(phpversion(), "4.3.0", ">=")) {
 // you're on 4.3.0 or later
} else {
 // you're not
}
?>

This little script can perhaps help you understand version comparison a little better - the output is displayed in the comment at the top. Tweak the list of versions if you need more examples...
<?php
# 1 lt 1.0
# 1.0 lt 1.01
# 1.01 eq 1.1
# 1.1 lt 1.10
# 1.10 gt 1.10b
# 1.10b lt 1.10.0
header('Content-type: text/plain');
$versions = array(
 '1',
 '1.0',
 '1.01',
 '1.1',
 '1.10',
 '1.10b',
 '1.10.0',
);
$comps = array(
 -1 => 'lt',
 0 => 'eq',
 1 => 'gt'
);
foreach ($versions as $version) {
 if (isset($last)) {
 $comp = version_compare($last, $version);
 echo str_pad($last,8,' ',STR_PAD_LEFT) . " {$comps[$comp]}{$version}\n";
 }
 $last = $version;
}
?>

Since this function considers 1 < 1.0 < 1.0.0, others might find this function useful (which considers 1 == 1.0):
<?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;
}
?>

Please note that supplying an operator that is not listed (e.g. ===), this function returns NULL instead of false.
Tested on PHP5.3.0, Win32

Note that both leading and trailing spaces on your version numbers can break version_compare(). 
Tested on PHP 5.6.8:
 <?php
 echo "\nShould be 0";
echo "\n '1.0.0-pl' vs. '1.0.0-pl' ---> " . version_compare('1.0.0-pl', '1.0.0-pl');
echo "\n '1.0.0-pl' vs. ' 1.0.0-pl' ---> " . version_compare('1.0.0-pl', ' 1.0.0-pl');
echo "\n ' 1.0.0-pl' vs. '1.0.0-pl' ---> " . version_compare(' 1.0.0-pl', '1.0.0-pl');
echo "\n '1.0.0-pl' vs. '1.0.0-pl ' ---> " . version_compare('1.0.0-pl', '1.0.0-pl ');
echo "\n '1.0.0-pl ' vs. '1.0.0-pl' ---> " . version_compare('1.0.0-pl ', '1.0.0-pl');
echo "\n\nShould be 1";
echo "\n '1.1.1-pl' vs. '1.0.0-pl' ---> " . version_compare('1.1.1-pl', '1.0.0-pl');
echo "\n ' 1.1.1-pl' vs. '1.0.0-pl' ---> " . version_compare(' 1.1.1-pl', '1.0.0-pl');
echo "\n\nShould be -1";
echo "\n '1.0.0-pl' vs. '1.1.1-pl' ---> " . version_compare('1.0.0-pl', '1.1.1-pl');
echo "\n '1.0.0-pl' vs. ' 1.1.1-pl' ---> " . version_compare('1.0.0-pl', ' 1.1.1-pl');
 /* Output 
Should be 0
 '1.0.0-pl' vs. '1.0.0-pl' ---> 0
 '1.0.0-pl' vs. ' 1.0.0-pl' ---> 1
 ' 1.0.0-pl' vs. '1.0.0-pl' ---> -1
 '1.0.0-pl' vs. '1.0.0-pl ' ---> 1
 '1.0.0-pl ' vs. '1.0.0-pl' ---> -1
Should be 1
 '1.1.1-pl' vs. '1.0.0-pl' ---> 1
 ' 1.1.1-pl' vs. '1.0.0-pl' ---> -1
Should be -1
 '1.0.0-pl' vs. '1.1.1-pl' ---> -1
 '1.0.0-pl' vs. ' 1.1.1-pl' ---> 1
 */

Something that may trip some folks up, but is useful to mention is that the following version comparison does not work quite as I expected:
 version_compare('1.0.1', '1.0pl1', '>') 
However, its quite easy to get working:
 version_compare('1.0.1', '1.0.0pl1', '>')

In case you need to handle more complex version numbers like alpha, beta,..., here is a little something that can help:
function multiExplode($delimiters,$string){
 $pattern = '/['.preg_quote($delimiters).']/';
 return(preg_split( $pattern, $string ));
}
function isInteger($input){
 return(ctype_digit(strval($input)));
}
function keepIntsStartArray($arr){
 $nonIntMappings=[
 "alpha"=>1,
 "beta"=>2,
 "gamma"=>3,
 "delta"=>4,
 "epsilon"=>5,
 "zeta"=>6,
 "eta"=>7,
 "theta"=>8,
 "iota"=>9,
 "kappa"=>10,
 "lambda"=>11,
 "mu"=>12,
 "nu"=>13,
 "xi"=>14,
 "omicron"=>15,
 "pi"=>16,
 "rho"=>17,
 "sigma"=>18,
 "tau"=>19,
 "upsilon"=>20,
 "phi"=>21,
 "chi"=>22,
 "psi"=>23,
 "omega"=>24,
 ];
 $ret=[];
 foreach($arr as $i){
 if(!isInteger($i)){
 if(!array_key_exists($i,$nonIntMappings)){
 break;
 }
 $ret[]=$nonIntMappings[$i];
 }
 $ret[]=$i;
 }
 return($ret);
}
function appVersionBigger($v1,$v2,$orEqual=false){
 $delimiters=".-+";
 $a1=keepIntsStartArray(multiExplode($delimiters,$v1));
 $a2=keepIntsStartArray(multiExplode($delimiters,$v2));
 $len=count($a1);
 if($len>count($a2)){
 $len=count($a2);
 }
 for($i=0;$i<$len;$i++){
 $n1=$a1[$i];
 $n2=$a2[$i];
 if($n1>$n2){
 return(true);
 }
 if($n1<$n2){
 return(false);
 }
 }
 if(count($a1)>count($a2)){
 return(true);
 }
 if(count($a1)<count($a2)){
 return(false);
 }
 return($orEqual);
}
// usage examples:
$versions=[
 "1.2.3-45"=>"1.2.3-45",
 "1.2.3-44"=>"1.2.3-45",
 "1.2.3-46"=>"1.2.3-45",
 "1.2.3"=>"1.2.3-45",
 "1.2.4"=>"1.2.3-45",
 "1.2.2"=>"1.2.3-45",
 "1.2"=>"1.2.3-45",
 "1.3"=>"1.2.3-45",
 "1.2.3-ios"=>"1.2.3-and",
 "1.2-ios"=>"1.2.3-and",
 "2-ios"=>"1.2.3-and",
 "1.2.3-alpha"=>"1.2.3-beta",
 "1.2.3-beta"=>"1.2.3-alpha",
 "1-gamma"=>"1.2.3-beta",
 "1-alpha"=>"1.2.3-beta",
];
foreach($versions as $v1=>$v2){
 echo("v1: ".$v1."\tv2: ".$v2."\tgt: ".(appVersionBigger($v1,$v2,false)?"true":"false")."\tge: ".(appVersionBigger($v1,$v2,true)?"true":"false")."\n");
}
// output with php version 8.1.8:
/*
v1: 1.2.3-45 v2: 1.2.3-45 gt: false ge: true
v1: 1.2.3-44 v2: 1.2.3-45 gt: false ge: false
v1: 1.2.3-46 v2: 1.2.3-45 gt: true ge: true
v1: 1.2.3 v2: 1.2.3-45 gt: false ge: false
v1: 1.2.4 v2: 1.2.3-45 gt: true ge: true
v1: 1.2.2 v2: 1.2.3-45 gt: false ge: false
v1: 1.2 v2: 1.2.3-45 gt: false ge: false
v1: 1.3 v2: 1.2.3-45 gt: true ge: true
v1: 1.2.3-ios v2: 1.2.3-and gt: false ge: true
v1: 1.2-ios v2: 1.2.3-and gt: false ge: false
v1: 2-ios v2: 1.2.3-and gt: true ge: true
v1: 1.2.3-alpha v2: 1.2.3-beta gt: false ge: false
v1: 1.2.3-beta v2: 1.2.3-alpha gt: true ge: true
v1: 1-gamma v2: 1.2.3-beta gt: true ge: true
v1: 1-alpha v2: 1.2.3-beta gt: false ge: false
*/

While `+` is typically used to append metadata that are not supposed to be accounted for version comparison, the PHP version_compare function will actually consider that it decreases the version (by less than the least significant number) so:
```
version_compare('1.0.0+something', '1.0.0', '>=')
```
will return `false`

Actually, it works to any degree:
<?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).

If you're careful, this function actualy works quite nicely for comparing version numbers from programs other than PHP itself. I've used it to compare MySQL version numbers. The only issue is that version_compare doesn't recognize the 'gamma' addition that mysql uses as being later than 'alpha' or 'beta', because the latter two are treated specially. If you keep this in mind though, you should have no problems.