php计算字符串有多少个a字符

<?php
header('content-type:text/html;charset=utf-8');   
$str="absjdnsjaajsaa";
echo "原字符串：".$str."
";
echo "统计a字符的个数：".substr_count($str,"a");
?>

<?php
header('content-type:text/html;charset=utf-8');   
$str="absjdnsjaajsaa";
echo "原字符串：".$str."
";
$con=0;
for($i=0;$i>strlen($str);$i++){
    if($str[$i]==="m"){
        $con++;
    }
}
echo "统计a字符的个数：".$con;
?>