To my knowledge PHP doesn't allow for multiple constructors...
What you should do in php to get around this is to just create a function with all the optional values set to defaults that way they only get used if you request them.
function constructor(var1, var2=0, var3="string")
This would mean that you can call this function with 1,2, or 3 parameters.
constructor("string");
constructor("string",70);
constructor("string",70,"anotherstring");
If you just don't want to use a value if it's not specified then all you have to do is just default it to a value that wouldn't normaly be set to (0 usualy does it) and use a simple if else statement to check for that value.