using single and double quotes in php form - php

Question

I ran into this code:
function input_text($elem, $val) {
print '<input type = "test" name="' . $elem .'" val="';
print htmlentities($val[elem]) . '"/>';
I m confused about the code: name="' . $elem .'" val="';
print htmlentities($val[elem]) . '"/>'
1) why put single quotes and dot inside double quotes around $elem? Can i just use double quotes like name="$elem".
2) what is the meaning of these code: val="';
print htmlentities($val[elem]) . '"/>'

Answer 1

1) Since the string being printed is surrounded with single quotes, variables are not expanded inside it; variables are only expanded inside double-quoted strings. So concatenation is necessary. If you change it to use double quotes, you could do variable interpolation:
print "<input type='test' name='$elem' val='";
2) There's no special meaning to it. The programmer simply chose to split up the commands to print this piece of HTML into two PHP print statements. So first he prints val=", then he prints htmlentities($val[elem]) . "">>'
The function could be rewritten as:
function input_text($elem, $val) {
print "<input type='test' name='$elem' val='" . htmlentities($val[elem]) . "'/>";
}
You have to use concatenation around htmlentities() -- only variables can be interpolated into strings, not function calls. However, you could assign the value to a variable first if you want:
function input_text($elem, $val) {
$valent = htmlentities($val[elem]);
print "<input type='test' name='$elem' val='$valent'/>";
}
BTW, $val[elem] looks like a typo, it probably should be $val[$elem].

Answer 2

The single quotes in this case denote a string in PHP.
$var = 'This is a String';
The reason they are used in conjunction with the double quotes is because the double quotes must be printed to get the correct HTML output of
<input type="test" name="someName" val="someValue" />
The . operator in PHP is the concatenation operator meaning combine 2 strings into 1.
$var = 'This' . ' and that'; //Evaluates to 'This and that'