ESP32 Using placeholders in HTML on SD Card

I have a working ESP32 WiFi sketch with index on SD card.
The WiFi operates in areas where there is no internet.
Now I need to place real time readings in the HTML.
I have spent extensive research trying to using place_holder to transfer temperature readings into the HTML to display the readings.

Extract of my HTML code is :
Ambient %PLACEHOLDER_TEMPERATURE1%
However this is the same that it displays the web page.

The answer might be in ESP32 Arduino HTTP Server: Template processing with multiple placeholders - techtutorialsx
but much of it is too advanced for me.

Here is my sketch as far as I can get based on the above Techtutorialsx page with a random string for test purposes.

I have a working ESP32 WiFi sketch with index on SD card.
Now I need to place real time readings in the HTML.
I have spent extensive research trying to using place_holder to transfer temperature readings into the HTML to display the readings.

Extract of my HTML code is :
li> Ambient %PLACEHOLDER_TEMPERATURE1%
However this is the same that it displays the web page.

The answer might be in ESP32 Arduino HTTP Server: Template processing with multiple placeholders - techtutorialsx
but much of it is too advanced for me.

Here is my sketch as far as I can get based on the above Techtutorialsx page with a random string for test purposes.

#include <Arduino.h>
#include <WiFi.h>
#include <AsyncTCP.h>
#include <ESPAsyncWebServer.h>
#include "FS.h"
#include "SD.h"
#include "SPI.h"

// network credentials
const char* ssid = "xxxxxxx";
const char* password = "13565564";
const char * html = "

Temperature: %PLACEHOLDER_TEMPERATURE1% ºC

";

// Create AsyncWebServer object on port 80
AsyncWebServer server(80);
String processor(const String& var)
{

Serial.println(var);

var == "PLACEHOLDER_TEMPERATURE1") {
return String(random(10, 20));
}

return String();
}
void initSDCard() {
if (!SD.begin()) {
Serial.println("Card Mount Failed");
return;
}
uint8_t cardType = SD.cardType();

if (cardType == CARD_NONE) {
Serial.println("No SD card attached");
return;
}

Serial.print("SD Card Type: ");
if (cardType == CARD_MMC) {
Serial.println("MMC");
} else if (cardType == CARD_SD) {
Serial.println("SDSC");
} else if (cardType == CARD_SDHC) {
Serial.println("SDHC");
} else {
Serial.println("UNKNOWN");
}
uint64_t cardSize = SD.cardSize() / (1024 * 1024);
Serial.printf("SD Card Size: %lluMB\n", cardSize);
}

void initWiFi() {
WiFi.mode(WIFI_STA);
WiFi.begin(ssid, password);
Serial.print("Connecting to WiFi ..");
while (WiFi.status() != WL_CONNECTED) {
Serial.print('.');
delay(1000);
}
Serial.println(WiFi.localIP());
}
void setup() {
Serial.begin(115200);
initWiFi();
initSDCard();

server.on("/sensor", HTTP_GET, (AsyncWebServerRequest * request) {
request->send(SD, "/index.html", "text/html");
});
server.serveStatic("/", SD, "/");
server.begin();
}

void loop() {
}

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