Can't set variable in string - php

Question

So I'm trying to send a dynamic email with PHP. Now here's what I have
$postString = '{
"key": "xxx",
"message": {
"html": "this is the emails html content",
"text": "this is the emails text content",
"subject": "this is the subject",
"from_email": "email#email.com",
"from_name": "Joe",
"to": [
{
"email": "Joe# Joe",
"name": "Joe# Joe"
}
],
"attachments": [
]
},
"async": false
}';
Now I want "html" to be a variable. So I did this
"html": $var,
Sadly that doesn't work. Not does {} or using single quotes. Any ideas? It gets picked up as a string, by the way.

Answer 1

Variables are not interpolated in strings delimited by single quotes. There are a few ways around this. The following example uses concatenation.
$postString = '{
"key": "xxx",
"html": "' . $var . '",
"message": {
"html": "this is the emails html content",
"text": "this is the emails text content",
"subject": "this is the subject",
"from_email": "email#email.com",
"from_name": "Joe",
"to": [
{
"email": "Joe# Joe",
"name": "Joe# Joe"
}
],
"attachments": [
]
},
"async": false
}';
To be honest with you, this would be a lot easier if you just used an array and then encoded it into JSON using json_encode().

Answer 2

As mentioned in my comment, this would work a lot better
$post = [
'key' => 'xxx',
'message' => [
'html' => $var,
'text' => 'this is the emails text content',
'subject' => 'this is the subject',
'from_email' => 'email#email.com',
'to' => [
['email' => 'Joe# Joe', 'name' => 'Joe# Joe']
],
'attachments' => []
],
'async' => false
];
$postString = json_encode($post);
Obligatory legacy PHP note: If you're stuck on a PHP version lower than 5.4, you obviously can't use the shorthand array notation. Substitute [] with array() if that's the case.