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Wait for a specific element before capturing in PHP with cURL extension

A fixed delay is a guess: too short and you capture the spinner, too long and every render pays for the worst case. wait_for_selector replaces the guess with a condition — pass a CSS selector and the render proceeds the moment a matching element is visible. The dashboard's chart, the map's tiles, the price table: name the thing you are actually waiting for, and wait for that.

Below is a complete, runnable PHP program using cURL extension, no third-party dependency needed. It reads your API key from the SNAPDOK_KEY environment variable — free keys take about thirty seconds and need no card.

<?php
$body = json_encode([
    'url' => "https://your-app.com/report/42",
    'format' => "png",
    'wait_for_selector' => "#chart-ready",
    'wait_until' => "networkidle",
]);

$ch = curl_init("https://snapdok.io/v1/render");
$headers = [];
curl_setopt_array($ch, [
    CURLOPT_POST => true,
    CURLOPT_POSTFIELDS => $body,
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_TIMEOUT => 90,
    CURLOPT_HTTPHEADER => [
        'Authorization: Bearer ' . getenv('SNAPDOK_KEY'),
        'Content-Type: application/json',
    ],
    CURLOPT_HEADERFUNCTION => function ($ch, $line) use (&$headers) {
        $parts = explode(':', $line, 2);
        if (count($parts) === 2) $headers[strtolower(trim($parts[0]))] = trim($parts[1]);
        return strlen($line);
    },
]);

$bytes = curl_exec($ch);
$status = curl_getinfo($ch, CURLINFO_RESPONSE_CODE);
curl_close($ch);

if ($status !== 200) {
    throw new RuntimeException("snapdok error $status: $bytes");
}
file_put_contents('ready.png', $bytes);
echo ($headers['x-cache'] ?? '?') . " // the settled render is cached like any other\n";

Selector choice is the whole game. Pick an element that appears last in your page's load sequence — a #chart-ready marker your own code adds after drawing, the final list item, the element a skeleton placeholder is replaced by. An element that exists in the initial HTML matches immediately and waits for nothing; visibility is required precisely so a pre-rendered-but-hidden node does not count as ready.

The failure mode is the feature: if the element never becomes visible within timeout, the render fails with 504 SELECTOR_TIMEOUT — not metered, and unambiguous in your logs — instead of silently shipping a half-rendered image the way an expired delay does. The sequencing composes with the other waits: navigation finishes (wait_until), then delay runs, then the selector wait begins. It works on every format, pdf_forms included.

Same endpoint, one more trick: if the page you are rendering has a form on it, adding "pdf_forms": true to a PDF render brings it back with real, fillable AcroForm fields — a PDF people can type into, not a picture of one. How that works.

Notes for cURL extension

Plain ext-curl, no Composer needed. The details that bite: CURLOPT_RETURNTRANSFER must be set or the bytes are echoed to stdout instead of returned; the HTTP status has to be read explicitly with curl_getinfo() because cURL happily "succeeds" on a 401; and response headers need a CURLOPT_HEADERFUNCTION callback if you want to check things like X-Form-Fields. file_put_contents is binary-safe in PHP — strings are byte arrays — so no "b" flag dance is needed.

Response headers worth reading

Header	Meaning
`X-Cache`	HIT when an identical request rendered within 24h.
`X-RateLimit-Remaining`	Requests left in the current second.

Full parameter reference: the docs. Hard numbers on caps and timeouts: limits.

Wait for a specific element before capturing in PHP with cURL extension

Notes for cURL extension

Response headers worth reading

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