PHP构造函数

<?php
class a{
function a(){
echo "i am the constructor of a.<br>\n";
}
function b(){
echo "i am a regular function named b in class a.<br>\n";
echo "i am not a constructor in a.<br>\n";
}}
class b extends a
{
function c(){
echo"i am a regular function.<br>\n";
}}
$b=new b;
?>
为什么最后只输出“i am the constructor of a.”这些函数是怎样调用的？
怎么说class b中没有构造函数，什么是构造函数？

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