difference between " quotes and '' quotes

[arvind vohra]

please define me what is the main difference between double quotes and single quotes ?

$ans=20;

echo "$ans";
output 20

echo '$ans';
output $ans;

if we can type


function update_query($filename,$data,$where)
	{
		$sql= "UPDATE ".$filename." SET ";		
		foreach ($data as $key=>$value)
		{
			$sql.=$key."='$value'";             
		}
		
		$sql = $sql." WHERE ".$where;
		echo " ".$sql."<br>"; die;
		return $res = $this->query($sql);  
	}

please define me that line

$sql.=$key."='$value'";

why we put "''" value in quotes .

because "" is used for print the value

and single quotes print the variable

but why used here single quotes for print the value and how print the value

That function is running condition.

[Rabbit]

Single quotes don't print variables. Double quotes will expand variables. Hence the reason the string is surrounded by double quotes. The single quotes are for the benefit of the SQL engine. SQL requires strings to be surrounded by single quotes, which is why those single quotes surround the value.

[Dormilich]

you'll find an in-depth explanation in the manual.