Probá así:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<style type="text/css">
.oc{
width:400px;
height:200px;
background:steelblue;
color:#fff;
display:none;
}
</style>
<script type="text/javascript">
var suma = 10;
function aparecer(){
this.intervalo=1;
var obj = this;
this.color= this.color || 0;
obj.style.display = "block";
this.color += suma;
if (!(this.color>=110)){
obj.style.filter = 'alpha(opacity='+this.color+')';
obj.style.opacity = this.color /100;
obj.style.MozOpacity = this.color /100;
obj.style.KHTMLOpacity = this.color /100;
window.setTimeout (function(){aparecer.call(obj);}, 100);
}else{
this.intervalo=0;
}
}
function desvanecer() {
this.intervalo=1;
var obj2 = this;
this.color= this.color || 0;
this.color -= suma;
if (!(this.color <=-10)){
obj2.style.filter = 'alpha(opacity='+this.color+')';
obj2.style.opacity = this.color /100;
obj2.style.MozOpacity = this.color /100;
obj2.style.KHTMLOpacity = this.color /100;
window.setTimeout (function(){desvanecer.call(obj2);}, 100);
}else{
obj2.style.display = "none";
this.intervalo=0;
}
}
function llama(){
if(this.intervalo)return;
if (!this.color || this.color<0){
aparecer.call(this);
}
else {
desvanecer.call(this);
}
}
</script>
</head>
<body>
<span onclick="llama.call(document.getElementById('escondido'));">Click Aqui</span>
<div class="oc" id="escondido">
<p>Lorem ipsum vel commune praesent maiestatis in, no suavitate
persecuti sententiae ius, ea cum facete sanctus detracto. Saepe
recteque tractatos qui te, ius summo mentitum at. Sed etiam option
delicatissimi ad. Alia altera lobortis vim id, id iisque commune
cum.</p>
</div>
<br />
<span onclick="llama.call(document.getElementById('escondido2'));">Click Aqui</span>
<div class="oc" id="escondido2">
<p>Lorem ipsum vel commune praesent maiestatis in, no suavitate
persecuti sententiae ius, ea cum facete sanctus detracto. Saepe
recteque tractatos qui te, ius summo mentitum at. Sed etiam option
delicatissimi ad. Alia altera lobortis vim id, id iisque commune
cum.</p>
</div>
</body>
</html>