The difference in results is as a result of a logic error in the php code.

The first call sends only 1 parameter.
The second call sends 6 parameters.
<?php

    $solrClient = new SolrClient(parse_ini_file('bdq/bdq/application/configs/solr.ini'));
    $solrQuery = new SolrQuery;
    $solrQuery->setQuery('l?gislative');
    // At this point only the 1 parameter has been set
    $response = $solrClient->query($solrQuery);
    
    $solrQuery->setShowDebugInfo(true);
    $solrQuery->setHighlight(true);
    $solrQuery->addHighlightField('questionsLabelsFr');
    $solrQuery->addHighlightField('variableLabelsFr');
    $solrQuery->setHighlightFragsize(0);
    var_dump($response);
    
    $solrQuery = new SolrQuery;
    $solrQuery->setQuery('l?gislative');
    $solrQuery->setShowDebugInfo(true);
    $solrQuery->setHighlight(true);
    $solrQuery->addHighlightField('questionsLabelsFr');
    $solrQuery->addHighlightField('variableLabelsFr');
    $solrQuery->setHighlightFragsize(0);
    
    // At this point 6 parameters have been set.
    $response = $solrClient->query($solrQuery);
    var_dump($response);

You're right about the logic error.
It was late in the day and I couldn't concentrate to give you a proper example.
I'm at home and I have not solr installed ATM.
Does it work this way ?

<?php
	$solrClient = new
SolrClient(parse_ini_file('bdq/bdq/application/configs/solr.ini'));
	$solrQuery = new SolrQuery;
	$solrQuery->setQuery('l?gislative');
	$solrQuery->setShowDebugInfo(true);
	$solrQuery->setHighlight(true);
	$solrQuery->addHighlightField('questionsLabelsFr');
	$solrQuery->addHighlightField('variableLabelsFr');
	$solrQuery->setHighlightFragsize(0);
	$response = $solrClient->query($solrQuery);
	var_dump($response);

	$solrQuery = new SolrQuery;
	$solrQuery->setQuery('l?gislative');
	$solrQuery->setShowDebugInfo(true);
	$solrQuery->setHighlight(true);
	$solrQuery->addHighlightField('questionsLabelsFr');
	$solrQuery->addHighlightField('variableLabelsFr');
	$solrQuery->setHighlightFragsize(0);
	$response = $solrClient->query($solrQuery);
	var_dump($response);

by the way, you can see in the object dump that the q=l?gislative appears twice, so there is a bug man

- First response :
protected 'http_raw_request' => string 'q=l%E9gislative'

- Second response :
protected 'http_raw_request' => string
'q=l%E9gislativeq=l%E9gislative&debugQuery=true&hl=true&hl.fl=questionsL
abelsFr%2CvariableLabelsFr&hl.fragsize=0' (length=111)

Can't you see in the second response q=l%E9gislativeq=l%E9gislative ???? Is it the way it is supposed to work ?

I'm gonna write my own client in PHP and come back to yours latter.

Xavier,

I found the problem.

The request data from the previous request was carried over in the buffer and was not reset.

So that is why the requests got concatenated.

I will fix it today and release it sometime tomorrow.

This bug has been fixed in SVN.

In case this was a documentation problem, the fix will show up at the
end of next Sunday (CET) on pecl.php.net.

In case this was a pecl.php.net website problem, the change will show
up on the website in short time.
 
Thank you for the report, and for helping us make PECL better.

This issue has been resolved in SVN revision number 293379.

The SolrClient request buffer was carrying over that from the previous requests without discarding it prior to building the new request data.

This fix will be available in the next release of the PECL extension.