php syntax..giving error

$sql='UPDATE Reg_Stud SET Result=$perc WHERE RegI开发者_如何转开发D="$_SESSION['id']"';

Whts wrong with this syntax

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Two problems:

• Variable interpolation does not happen in single quotes.
• An un-escaped quote in a string prematurely terminates the string.

You can do:

$sql='UPDATE Reg_Stud SET Result='.$perc.' WHERE RegID='.$_SESSION['id'];