$proxy[0]['host'] = '***';
$proxy[0]['port'] = 3128;
$proxy[1]['host'] = '***';
$proxy[1]['port'] = 3128;
$proxy[2]['host'] = '69.72.144.34';
$proxy[2]['port'] = 3128;
$proxy[3]['host'] = '114.30.47.10';
$proxy[3]['port'] = 80;

$prn=0;//по большому счету рандом или последовательно
$proxy=$proxy[$prn]['host'];
$prprt=$proxy[$prn]['port'];

Cannot use string offset as an array