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The single and double quotation marks in PHP

One of the most annoying stuffs in PHP for a newbie like me is the use of single quotation marks ' and double quotation marks''. When should I use ' or for "?
Here's a example,

$id = 1;
$ok1 = @mysql_query("DELETE from Authors where ID=$id");

We can only use '' here , because only the variables quoted within '' can be interpreted by PHP.If we replace '' by ' , then the actual value we passed in the query will be '$id', not 1.

Lets see another example,

In this echo clause, we the variables $name, $id need to be interpreted by PHP, therefore we can't use sigle quotation, because PHP won't interpret variables inside signle quotation marks.

However, we also notice that the two $id are indeed quoted within single qoatation marks, so will they be interpreted by PHP?
The answer is yes.It seems like the we can use '' and ' together when we need nested quotations,and PHP will still interpret the varibles inside single quotions marks if they are within outer double quotions marks.

you can also see this from ,