Update php.md

Author: cswatt

content/en/tracing/setup_overview/setup/php.md

@@ -213,7 +213,7 @@ DD_TRACE_DEBUG=true php -S localhost:8888
 | `DD_TRACE_RESOURCE_URI_MAPPING_INCOMING` | `null` | CSV of URI mappings to normalize resource naming for incoming requests (see [Map resource names to normalized URI](#map-resource-names-to-normalized-uri)). |
 | `DD_TRACE_RESOURCE_URI_MAPPING_OUTGOING` | `null` | CSV of URI mappings to normalize resource naming for outgoing requests (see [Map resource names to normalized URI](#map-resource-names-to-normalized-uri)). |
 | `DD_TRACE_SAMPLE_RATE` | `1.0` | The sampling rate for the traces (defaults to: between `0.0` and `1.0`). For versions < `0.36.0`, this parameter is `DD_SAMPLING_RATE`. |
-| `DD_TRACE_SAMPLING_RULES` | `null` | A JSON encoded string to configure the sampling rate. Examples: Set the sample rate to 20%: `'[{"sample_rate": 0.2}]'`. Set the sample rate to 10% for services starting with 'a' and span name 'b' and set the sample rate to 20% for all other services: `'[{"service": "a.*", "name": "b", "sample_rate": 0.1}, {"sample_rate": 0.2}]'` (see [Integration names](#integration-names)). Note that the json object has to be included in single quotes `'` to avoid problems with escaping of the `"` character.|
+| `DD_TRACE_SAMPLING_RULES` | `null` | A JSON encoded string to configure the sampling rate. Examples: Set the sample rate to 20%: `'[{"sample_rate": 0.2}]'`. Set the sample rate to 10% for services starting with 'a' and span name 'b' and set the sample rate to 20% for all other services: `'[{"service": "a.*", "name": "b", "sample_rate": 0.1}, {"sample_rate": 0.2}]'` (see [Integration names](#integration-names)). Note that the JSON object **must** be included in single quotes (`'`) to avoid problems with escaping of the double quote (`"`) character.|
 | `DD_TRACE_URL_AS_RESOURCE_NAMES_ENABLED` | `true` | Enable URL's as resource names (see [Map resource names to normalized URI](#map-resource-names-to-normalized-uri)). |
 | `DD_VERSION` | `null` | Set an application’s version in traces and logs, for example: `1.2.3`, `6c44da20`, `2020.02.13`. Added in version `0.47.0`. |