function startAPP(){ window.location = "**://**.**/**"; } function downloadAPP(){ window.location = "http://IP/**/**.apk"; } 

<span style="font-size:18px;"><script>
function startAPP(){
window.location = "scheme name://**.**/**";
}
function downloadAPP(){
window.location = "http://IP/**/**.apk";
}

</script>
<style type="text/css">
        body{background:#D2D460;text-align:center;margin-top:10%}
        div{width:100%;margin:0 auto;background:#fff;text-align:left;}
</style>
</head>
<body>

<?php 
$var_name =  $_GET["isappinstalled"]; 
if ($var_name=="1"){
    echo '<img src=images/start.png alt=打开APP onclick=startAPP() ></img>';
}else if ($var_name=="0"){
    echo '<img src=images/download.png alt=打开APP onclick=downloadAPP() ></img>';
}else{
    echo '<img src=images/welcome.png alt=打开APP onclick=startAPP() ></img>';
}
?>

</body></span>

在app里面需要做以下设置（一般是在manifest.xml文件的activity的 intent filter里面）

<span style="font-size:18px;"><intent-filter>
               <action android:name="android.intent.action.VIEW" />
               <category android:name="android.intent.category.DEFAULT" />
               <category android:name="android.intent.category.BROWSABLE" />
               <data 
            android:scheme="**" //scheme name 'TencentQQ'
            android:host="**.**" 
            android:pathPrefix="/**">
         </data>
</intent-filter></span>