On Mac OSX, to see if a file is a FInder alias:
<?PHP
if( getFinderAlias( $someFile , $target ) ) {
 echo $target;
}
else {
 echo "File is not an alias";
}
function getFinderAlias( $filename , &$target ) {
$getAliasTarget = <<< HEREDOC
    -- BEGIN APPLESCRIPT --
    set checkFileStr to "{$filename}"
    set checkFile to checkFileStr as POSIX file
    try
        tell application "Finder"
            if original item of file checkFile exists then
                set targetFile to (original item of file checkFile) as alias
                set posTargetFile to POSIX path of targetFile as text
                get posTargetFile
            end if
        end tell
    end try
    -- END APPLESCRIPT --
HEREDOC;
$runText = "osascript << EOS\n{$getAliasTarget}\nEOS\n";
$target = trim( shell_exec( $runText ) );
return ( $target == "" ? false : true );
}
?>