You are looking for str_replace()
.
$string = 'blah blarh bleh bleh blarh';
$result = str_replace(
array('blah', 'blarh'),
array('bleh', 'blerh'),
$string
);
// Additional tip:
And if you are stuck with an associative array like in your example, you can split it up like that:
$searchReplaceArray = array(
'blah' => 'bleh',
'blarh' => 'blerh'
);
$result = str_replace(
array_keys($searchReplaceArray),
array_values($searchReplaceArray),
$string
);
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