php - insert a variable in an echo string

Always use double quotes when using a variable inside a string and backslash any other double quotes except the starting and ending ones. You could also use the brackets like below so it's easier to find your variables inside the strings and make them look cleaner.

$var = 'my variable';
echo "I love ${var}";

or

$var = 'my variable';
echo "I love {$var}";

Above would return the following: I love my variable

Variable interpolation does not happen in single quotes. You need to use double quotes as:

$i = 1
echo "<p class=\"paragraph$i\"></p>";
++i;

Single quotes will not parse PHP variables inside of them. Either use double quotes or use a dot to extend the echo.

$variableName = 'Ralph';
echo 'Hello '.$variableName.'!';

OR

echo "Hello $variableName!";

And in your case:

$i = 1;
echo '<p class="paragraph'.$i.'"></p>';
++i;

OR

$i = 1;
echo "<p class='paragraph$i'></p>";
++i;